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HOW  TO  CHOOSE  AND 
USE  A  LENS 


Practical  Photography,  No.  3 


EDITED  BY 

FRANK  R.  FRAPRIE,  S.  M.,  F.  R.  P.  S. 

Editor  of  AMERICAN  PHOTOGRAPHY  and  POPULAR  PHOTOGRAPHY 


American  Photographic  Publishing  Co, 

Boston,   Mass. 

1915 


Copyright,  1915, 

by 
AMERICAN  PHOTOGRAPHIC  PUBLISHING  Co. 


Stanbope  jprcss 

H.GILSON   COMPANY 
BOSTON,  U.S.A. 


HOW  TO  CHOOSE  AND 
USE  A  LENS 


Introductory.  —  The  purchase  of  a  camera  neces- 
sarily involves  the  choice  of  a  lens,  and  while  few 
beginners  in  photography,  whether  they  select  their 
cameras  for  themselves  or  acquire  them  by  gift, 
make  any  very  logical  decision  as  to  the  capabilities 
of  the  outfit  they  desire,  the  time  is  sure  to  come  to 
every  amateur  when  intelligent  knowledge  of  what 
a  lens  will  do  is  necessary.  We  will  assume  that  the 
reader  is  about  to  buy  a  camera,  and  that  he  knows 
nothing  about  photography.  We  will  endeavor  to 
show  him  what  a  lens  is,  and  what  each  type  now 
obtainable  will  do.  With  this  knowledge  he  can 
study  the  catalogues  of  lens  and  camera  manu- 
facturers with  more  confidence,  and  decide  for  him- 
self whether  the  expenditure  of  five  dollars  or  fifty 
dollars  for  a  camera  and  lens  making  a  given  size  of 
picture  is  best  for  his  necessities.  We  may  say, 
however,  that  practically  every  step  in  increased 
price  of  either  lens  or  camera  is  fully  justified  by 
advance  in  careful  workmanship,  increased  facility 
of  operation,  or  more  versatile  performance,  and  that 
the  higher-priced  outfits  in  case  of  forced  sale  usually 
bring  a  greater  percentage  of  original  cost.  He  who 
begins  with  an  outfit  of  limited  capacity  and  con- 

3 

328323 


-  TQ  CHOOSE 


tinues  in  photography  invariably  desires  a  more 
flexible  equipment.  We  therefore  unqualifiedly  ad- 
vise the  original  purchase  of  the  best  outfit  which 
the  buyer's  means  will  command,  as  the  most  satis- 
factory and  economical  course  in  the  end. 

In  the  majority  of  cases,  the  selection  of  one's 
first  lens  means  the  purchase  of  a  camera  complete 
and  ready  for  use,  but  this  scarcely  limits  the  scope 
for  choice,  for  every  grade  of  lens  may  be  had  today 
on  the  moderate  priced  cameras  of  all  makers,  and 
only  in  the  cheapest  styles  is  there  no  choice  of  lens 
equipment.  We  will  begin  with  a  consideration  of 
the  simplest  forms  of  lenses,  but  we  must  first  con- 
sider the  elementary  principles  of  optics. 

Light  and  Its  Transmission.  —  To  the  physicist, 
light  presents  itself  as  vibrations  of  the  luminiferous 
ether,  a  perfectly  elastic  medium  which  is  supposed 
to  fill  space,  and  the  principles  of  optics  are  deduced 
from  a  mathematical  consideration  of  the  changes  of 
direction  and  velocity  of  these  waves  or  vibrations 
when  passing  from  one  medium  to  another.  A  much 
simpler  conception  is  to  assume  that  every  luminous 
point  emits  rays  of  light  which  travel  in  absolutely 
straight  lines  in  every  possible  direction  as  long  as 
they  remain  in  the  same  medium.  In  passing  from 
a  rarer  medium,  such  as  air,  to  a  denser  one,  such  as 
glass,  the  speed  of  the  rays  is  lessened,  and  from 
this  fact  follows  the  possibility  of  a  lens,  as  will 
appear  later. 

That  light  travels  in  a  straight  line  may  be  easily 
observed.  Place  a  dark  screen  through  which  a  pin- 
hole  has  been  pierced  between  the  eye  and  a  distant 
arc  light.  But  one  position  of  the  screen  can  be 


AND  USE  A  LENS.  5 

found  through  which  the  light  will  be  visible,  and 
this  is  in  a  straight  line  between  the  eye  and  the 
light.  This  can  be  mathematically  proved,  as  can 
all  the  facts  about  light  and  lenses  which  will  follow, 
but  the  reader  who  wants  such  proof  must  seek  it  in 
more  extensive  treatises  than  this. 

Pinhole  Images.  —  If  we  make  a  pinhole  in  the 
light-tight  shutter  of  a  room  from  which  all  daylight 
is  excluded,  and  place  behind  this  a  white  card,  we 
will  see  upon  the  card  an  image  in  natural  colors  of 
outside  objects.  Observing  this,  we  will  see  that  the 
image  is  inverted,  the  sky  being  at  the  bottom,  and 
objects  at  the  right  outdoors  appearing  on  the  left 
side.  This  is  the  natural  and  only  possible  conse- 
quence of  the  fact  that  light  travels  in  straight  lines. 
Rays  from  the  top  and  bottom  of  any  object  con- 
verge at  the  pinhole,  cross  there,  and  reach  the  card 
to  form  the  inverted  image.  Lenses  also  cause  the 
rays  to  cross,  and  so  the  image  on  the  ground  glass 
of  the  camera  is  inverted. 

The  Pinhole  Camera.  —  The  beauty  of  such  an 
image  produced  in  a  dark  room  inspires  the  thought 
that  it  might  be  recorded  by  photography,  and  in 
fact  the  simplest  apparatus  for  making  photographs 
is  a  light-tight  box  which  contains  a  minute  opening 
in  the  centre  of  one  end.  At  the  opposite  end  a 
sensitive  plate  is  placed  to  receive  the  inverted  image 
of  external  objects  produced  by  this  minute  opening. 
The  nearer  the  plate  is  to  the  pinhole,  the  smaller 
the  image  of  any  given  external  object,  but  the 
larger  the  area  of  the  view  outside  which  is  recorded 
on  the  plate. 

The  sharpness  of  the  image  produced  in  a  pinhole 


6  HOW  TO  CHOOSE 

camera  depends  partly  on  the  fineness  of  the  aper- 
ture, and  partly  on  the  distance  of  the  plate  from  it, 
but  mainly  on  the  first.  If  the  aperture  could  be 
made  so  small  that  only  a  single  ray  without  thick- 
ness could  pass  through  it,  the  image  would  be  ab- 
solutely sharp.  A  decrease  of  the  area  of  the  aper- 
ture means  a  corresponding  loss  in  brightness  of  the 
image,  and  if  it  is  made  too  small  there  is  also  a  lack 
of  sharpness  from  the  phenomenon  known  as  dif- 
fraction, which  is  a  bending  from  their  course  of 
part  of  the  rays  of  light  which  strike  the  edge  of 
the  opening.  This,  though  small  in  amount  with  a 
large  opening,  deflects  an  appreciable  proportion  of 
the  light  passed  by  a  very  small  opening.  In  practice, 
therefore,  the  pinhole  must  have  a  fairly  large  di- 
ameter to  produce  an  image  bright  enough  to  impress  a 
plate  in  a  reasonable  time,  and  the  practical  problem 
is  to  find  the  mean  between  lack  of  sharpness  from 
diffraction  and  that  caused  by  too  large  an  aperture, 
which  allows  a  large  number  of  rays  from  any  given 
point  to  pass  through,  spreading  out  as  they  go  and 
causing  unsharp  contours.  At  best  the  pictures  made 
with  a  pinhole  camera  are  far  from  sharp,  and  the 
method  is  curious  rather  than  useful,  though  it  has 
been  much  practiced  in  recent  years,  and  several 
books  have  been  written  on  the  subject.  There  is 
considerable  skill  involved  in  making  a  successful 
pinhole,  and  those  interested  in  the  subject  would 
find  the  purchase  of  the  proper  apparatus  nearly  as 
cheap  as  its  manufacture. 

Pinhole  Attachments.  —  There  are  on  the  market 
several  styles  of  pinhole  attachments  which  may  be 
substituted  for  the  usual  lens  on  any  camera  from 


AND   USE  A  LENS.  7 

which  the  objective  is  detachable.  One  style,  selling 
at  sixty  cents,  has  an  opening  listed  as  No.  6  on  the 
scale  given  below.  A  more  expensive  form,  listing 
at  $2,  has  four  pinholes  and  an  aperture  for  the 
purpose  of  focusing.  The  holes  are  numbered  ac- 
cording to  the  system  devised  by  Alfred  Watkins  on 
D'Arcy  Power's  suggestion,  and  called  by  Watkins 
"  Watkins-Power "  numbers.  The  complete  list  is 
as  follows: 


W.-P.  No. 

Diameter  in  inches 

Nearest  Needle  No. 

Best  Distance  for  Plate 

3 

0-053 

No.    i 

40  inches 

4 

0.040 

No.    4 

20  inches 

5 

0.032 

No.    5 

15  inches 

6 

0.027 

No.    7 

10  inches 

7 

0.023 

No.    8 

8  inches 

8 

0.020 

No.  10 

5  inches 

10 

0.016 

No.  12 

32  inches 

12 

0.013 

No.  13 

2$  inches 

Using  the  W.-P.  Number.  —  Multiply  the  W.-P. 
number  by  the  distance  from  pinhole  to  plate  and 
use  the  result  as  the  /  value  of  the  pinhole.  Then 
whatever  the  exposure  would  be  for  that  /  number 
with  a  lens  in  seconds  or  fractions  thereof  is  the  ex- 
posure for  the  pinhole  in  minutes  or  fractions  thereof. 
For  example,  W.-P.  hole  No.  6  used  at  a  distance 
of  10  inches  from  the  plate  is  regarded  as  /:6o.  If 
the  exposure,  as  determined  by  The  American  Photog- 
raphy Exposure  Tables,  or  by  an  exposure  meter, 
for  a  lens  working  at  /:6o  under  the  given  conditions 
is  2  seconds,  then  2  minutes  should  be  given  for  the 
pinhole  exposure.  This  system  of  calculating  pinhole 
exposures  is  the  most  convenient  yet  devised. 


8  HOW  TO  CHOOSE 

Utility  of  the  Pinhole.  —  As  indicated  in  the  table 
given  above,  there  is  a  most  favorable  distance  for 
the  use  of  any  given  pinhole,  but  it  is  not  absolutely 
necessary  to  have  more  than  one,  and  a  No.  6  pinhole 
may  be  used  on  an  ordinary  camera  at  any  distance 
within  the  capacity  of  the  bellows,  with  results  vary- 
ing very  little.  The  pinhole  camera  finds  a  certain 
application  in  architectural  photography,  especially 
in  cramped  quarters,  for  it  may  be  used  to  cover  an 
extremely  wide  angle  if  the  plate  is  very  near  the 
opening.  The  image  is  perfectly  rectilinear.  Owing 
to  the  slight  diffusion  and  consequent  obliteration  of 
the  finer  detail  inseparable  from  pinhole  exposures, 
pictures  thus  made  are  much  in  vogue  among  workers 
of  artistic  temperament. 

Refraction  of  Light.  —  The  reason  that  we  cannot 
get  a  perfectly  sharp  image  with  a  pinhole  is  that,  as 
it  is  larger  than  a  mathematical  point,  the  rays  from 
any  point  in  the  object  form  a  cone  in  passing  through 
it,  and  continue  to  diverge  indefinitely.  We  there- 
fore need  some  means  of  once  more  bringing  these 
rays  back  to  a  point,  and  this  we  do  by  means  of  a 
lens,  the  utility  of  which  is  due  to  the  phenomenon 
known  as  refraction.  As  we  have  previously  stated, 
when  rays  of  light  pass  from  a  rarer  medium  to  a 
denser,  their  speed  is  retarded;  on  the  other  hand, 
the  velocity  is  increased  in  passing  from  a  denser 
medium  to  a  rarer  one.  When  the  passage  from  one 
medium  to  another  takes  place  in  a  direction  per- 
pendicular to  the  surface  dividing  the  two  media, 
there  is  no  change  in  the  direction  of  the  rays,  though 
their  velocity  is  changed.  When  the  direction  of  the 
rays  is  oblique  to  this  surface,  the  course  they  follow 


AND   USE  A  LENS.  9 

is  bent  or  refracted.  We  will  not  go  into  any  detailed 
explanation  of  the  laws  of  refraction;  it  will  be 
sufficient  for  us  to  know  at  present  that  when  a  ray 
of  light  passes  from  air  through  a  piece  of  glass  the 
sides  of  which  are  parallel,  the  direction  of  the  ray 
after  emerging  is  parallel  to  its  direction  when  it 
entered,  while  if  the  sides  of  the  glass  are  not  parallel, 
so  that  it  forms  a  wedge  or  prism,  the  direction  of 
the  ray  is  changed,  and  its  course  is  bent  or  refracted 
away  from  the  angle  of  the  prism  and  toward  its 
base.  If  we  take  two  prisms  and  set  them  base  to 
base,  it  is  evident  that  two  parallel  rays,  one  of 
which  passes  through  each  prism  and  is  duly  re- 
fracted toward  its  base,  must  eventually  cross,  and 
these  two  prisms  are  the  germ  of  the  lens.  Parallel 
rays  passing  through  one  of  these  two  prisms  would 
emerge  with  their  direction  changed,  but  still  parallel, 
and  would  cross  the  corresponding  rays  bent  by  the 
other  prism  at  variable  distances  from  the  prisms. 
The  amount  of  the  bending  depends  on  the  angle  in- 
cluded by  the  prism,  increasing  as  this  does,  and  so 
if  we  have  a  number  of  very  thin  slices  of  prisms  of 
continually  increasing  angles  piled  on  each  other,  we 
could  make  successive  parallel  rays  bend  more  and 
more  toward  the  base,  so  that  they  would  all  cross 
corresponding  rays  from  the  similar  prisms  on  the 
other  side  at  a  single  point.  Now,  an  ordinary  lens 
is  just  such  a  collection  of  portions  of  prisms,  for  all 
curved  surfaces  made  by  mechanical  means  are  com- 
posed of  small  surfaces  joining  each  other  at  large 
angles,  instead  of  being  truly  continuous  curves. 
Thus  we  have  found  that  by  placing  a  lens  of  glass  of 
the  proper  form  in  the  place  of  our  pinhole,  we  can 


io  HOW  TO  CHOOSE 

cause  the  rays  to  come  together  instead  of  separat- 
ing, and  it  is  a  matter  of  common  experience  that 
thereby  a  sharp  image  can  be  formed,  instead  of  the 
diffused  one  produced  by  the  pinhole. 

Simple  Lenses.  —  From  the  two  prisms  placed 
base  to  base  may  be  derived  three  forms  of  lenses,  all 
of  which  are  thicker  at  the  centre  than  at  the  edges. 
They  are  called  the  double  convex,  with  two  convex 
surfaces,  the  plano-convex,  with  one  convex  surface 
and  one  flat  surface,  and  the  convex  meniscus,  with 
one  convex  and  one  concave  surface,  but  thicker  at 
the  middle  than  at  the  edges.  All  of  these  forms  of 
lenses  cause  the  light  rays  to  converge  until  they 
meet  upon  the  axis  of  the  lens,  an  imaginary  line 
drawn  perpendicularly  through  the  centre  of  the  lens 
surfaces,  and  they  are  therefore  called  converging  or 
positive  lenses.  If  we  assume  that  the  two  prisms 
are  placed  together  point  to  point  instead  of  base  to 
base,  we  get  the  foundation  for  three  more  kinds  of 
lenses,  the  double  concave,  the  plano-concave,  and 
the  concave  meniscus.  All  these  lenses  are  thicker  at 
the  edges  than  in  the  middle,  cause  rays  of  light  to 
diverge  from  the  axis,  and  are  called  diverging  or 
negative  lenses.  These  six  forms  of  lenses  are  all 
used  in  photography,  and  from  them  in  combination 
all  types  of  photographic  lenses  are  constructed. 
The  curved  surfaces  which  bound  them  are  in  practice 
always  spherical,  for  in  the  process  of  grinding  it  is 
mechanically  easy  to  produce  nearly  perfect  spherical 
surfaces,  whereas  the  construction  of  a  lens  whose 
surfaces  should  be  sections  of  a  paraboloid,  hyper- 
boloid,  ellipsoid,  or  other  geometrical  solid  more  com- 
plex than  the  sphere,  while  not  impossible,  would  be 


AND   USE  A  LENS.  n 

a  very  expensive  and  laborious  task.  It  is  neverthe- 
less probable  that  lenses  with  surfaces  of  this  more 
complicated  description  will  be  produced  in  the 
future,  for  it  seems  impossible  at  the  present  time  to 
introduce  much  further  refinement  into  present  day 
lenses  that  are  made  with  spherical  surfaces  alone. 

Dispersion  of  Light.  —  We  have  now  seen  that  it 
is  possible  to  make  rays  of  light  cross  each  other  on 
the  axis  of  a  lens  by  the  use  of  a  double  convex  or 
other  converging  lens.  It  is,  however,  no  more  pos- 
sible to  make  a  perfect  and  satisfactory  photograph 
with  a  single  lens  of  this  character  than  we  have 
seen  it  to  be  with  a  pinhole.  In  order  to  understand 
the  most  elementary  reason  therefor,  we  must  revert 
to  our  illustration  of  the  prism.  In  considering  this, 
we  saw  that  when  a  ray  of  light  falls  upon  a  prism 
the  ray  is  bent  out  of  its  course.  A  further  observation 
can  be  made  of  the  effect  of  a  prism  upon  light.  If  a 
beam  of  sunlight  falls  upon  the  prism,  and  is  bent 
from  its  course,  upon  leaving  the  prism  it  no  longer 
appears  as  a  beam  of  white  light,  but  if  we  place 
beyond  the  prism  a  sheet  of  white  paper,  we  will  find 
upon  it  instead  of  white  light,  a  band  of  light  of 
different  colors.  In  other  words,  the  prism  has  split 
up  or  dispersed  the  beam  of  white  light  into  a  band  of 
colors  called  the  spectrum,  in  which  the  light  passes 
by  gradual  change  from  one  color  to  another,  in  the 
following  order:  violet,  blue,  green,  yellow,  orange, 
red.  The  order  of  these  colors  is  always  the  same, 
and  the  violet  ray  is  refracted  the  most  while  the  red 
ray  is  refracted  the  least. 

Chromatic  Aberration.  —  A  lens  of  course  acts  in 
the  same  way  as  the  prism  and  separates  the  white 


12  HOW  TO   CHOOSE 

rays  of  light  which  pass  through  it  into  their  colored 
components.  The  violet  rays  therefore  cross  on  the 
axis  of  the  lens  at  a  point  nearer  the  lens  than  do  the 
red  rays.  The  point  at  which  any  given  rays  come 
together  on  the  axis  of  the  lens  is  called  the  focus  of 
these  rays,  and  thus  in  the  use  of  a  simple  lens  we 
find  that  the  focus  of  the  violet  rays  is  nearer  the 
lens  than  the  focus  of  the  red  rays.  Photographically, 
this  is  unfortunate,  for  the  image  which  we  see  with 
the  eye  on  the  ground  glass  of  the  camera  is  composed 
mostly  of  yellow  rays,  which  have  the  greatest  visual 
luminosity,  that  is,  affect  the  eye  the  most.  On  the 
other  hand,  the  photographic  plate  is  most  affected 
by  the  violet  rays,  and  when  we  have  focused  on  the 
image  formed  by  the  yellow  rays,  and  put  a  plate  in 
place  of  the  ground  glass,  the  image  produced  by  the 
violet  rays  is  brought  to  a  focus  nearer  the  camera 
than  the  plate,  and  the  image  which  we  photograph 
is  out  of  focus,  diffused  or  blurred.  It  is  possible  by 
means  of  a  simple  calculation  to  find  out  how  much  it 
is  necessary  to  move  the  plate  after  focusing,  approxi- 
mately ^V  of  the  focal  length  of  the  lens,  in  order  to 
place  it  at  the  focus  of  the  violet  or  actinic  rays,  as 
they  are  called,  instead  of  at  the  focus  of  the  yellow 
rays  which  the  eye  easily  perceives,  but  this  is  a 
clumsy  manipulation,  and  a  simpler  method  has  been 
found  to  obviate  this  difficulty.  Opticians  have  dis- 
covered that  glasses  of  different  chemical  constitution 
have  different  dispersions,  that  is,  split  up  white 
light  to  different  extents,  and  that  by  the  combination 
of  a  positive  or  collecting  lens,  of  a  kind  of  glass 
known  as  crown,  with  a  negative  or  dispersing  lens  of 
another  kind  of  glass  known  as  flint,  of  a  different 


AND  USE  A  LENS.  13 

dispersive  power,  both  the  chemical  and  the  visual 
rays  can  be  brought  together  at  a  single  point.  When 
this  has  been  done,  the  lens  is  said  to  be  achromatic, 
and  practically  all  modern  lenses  are  achromatized 
for  two  points  in  the  spectrum,  one  in  the  yellow  and 
one  in  the  violet.  For  certain  purposes  it  is  neces- 
sary to  produce  lenses  of  finer  color  correction  than 
this,  and  lenses  for  three-color  work,  for  instance,  are 
corrected  so  that  there  is  a  common  focus  for  three 
different  points  in  the  solar  spectrum,  corresponding 
to  the  three  colors  chosen  to  produce  the  image. 
Such  lenses  are  known  as  apochromats. 

Spherical  Aberration.  —  If  we  were  to  separate 
from  the  solar  spectrum  light  of  a  single  color,  and 
allow  this  to  fall  upon  the  whole  surface  of  a  simple 
collecting  lens,  we  would  find  that  the  lens  would 
be  unable  to  bring  all  the  rays  of  light  of  a  single 
color  to  a  single  focus.  The  rays  passing  through  the 
edge  of  the  lens  would  be  bent  a  little  more  sharply 
and  would  come  to  a  crossing  on  the  axis  somewhat 
sooner  than  those  passing  through  the  lens  nearer  the 
centre.  This  property  is  inherent  in  all  lenses  having 
spherical  surfaces,  and  is  hence  called  spherical  aber- 
ration. It  may  be  partly  corrected  by  using  a 
diaphragm  which  will  cut  off  the  rays  of  light  passing 
through  the  outside  of  the  lens  and  allow  only  those 
striking  the  central  portion  to  be  used,  but  it  is 
more  completely  corrected  in  practice  by  combining 
a  positive  and  a  negative  lens,  of  opposing  spherical 
aberrations,  in  such  a  way  that  one  counteracts  the 
other.  Modern  glasses  are  of  such  diverse  proper- 
ties that  it  is  possible  to  correct  a  lens  simultane- 
ously for  chromatic  and  spherical  aberration.  When 


I4  HOW  TO  CHOOSE 

a  single  combination  has  been  so  corrected  that  it  will 
produce  sharp  focus,  with  a  comparatively  large  work- 
ing aperture,  it  is  said  to  be  aplanatic  or  an  aplanat. 

The  Achromatic  Meniscus  Lens.  —  The  simplest 
type  of  lens  used  in  photography  of  any  importance 
today  is  that  corrected  for  chromatic  aberration,  and 
known  as  the  achromatic  meniscus.  This  is  the  kind 
of  lens  which  is  used  in  most  of  the  cheap  so-called 
fixed-focus  cameras,  which  sell  from  $i  up.  Certain 
very  cheap  cameras  may  be  equipped  with  a  simple 
spectacle  lens,  but  they  are  of  so  little  photographic 
importance  that  this  type  of  lens  does  not  need  to 
be  considered  at  length.  The  achromatic  meniscus 
works  well  in  bright  summer  sunlight,  with  a  snap- 
shot shutter  working  at  about  ^  of  a  second  and 
gives  a  reasonably  well-defined  image  for  small  direct 
prints.  This  type  of  lens,  however,  though  partly 
corrected,  still  has  a  number  of  defects,  which  make 
it  impossible  to  obtain  minutely  sharp  definition  ex- 
cept with  a  very  small  stop,  and  even  if  such  stops 
are  provided  on  the  camera,  they  can  rarely  be  used 
except  in  the  very  brightest  light. 

Usefulness  of  the  Meniscus  Lens.  —  The  user  of  a 
cheap  type  of  camera  almost  invariably  desires  to  get 
a  large  section  of  the  view  before  him  on  his  film  or 
print,  and  for  this  reason  achromatic  meniscus  lenses 
are  usually  of  very  short  focus,  thereby  taking  in  a 
large  angle  of  view.  As  a  result  of  this,  and  the  ad- 
ditional fact  that  the  lens  is  stopped  down  so  as  to 
utilize  only  the  centre  and  best  working  part  of  the 
lens,  a  camera  of  this  type  will  give  a  fairly  sharp 
image  of  objects  from  about  eight  feet  from  the 
camera  to  the  limit  of  visibility,  without  any  pro- 


AND  USE  A  LENS.  15 

vision  for  focusing.  Consequently  such  cameras  are 
usually  called  "  fixed-focus  "  cameras,  a  term  evi- 
dently designed  by  some  advertising  man  to  make 
capital  from  the  fact  that  it  is  not  necessary  to  focus 
such  cameras  for  ordinary  landscape  pictures.  Prob- 
ably the  term  "  average-focus  "  would  be  a  better 
one  for  a  camera  of  this  type,  but  whether  the  name 
be  proper  or  not,  such  instruments  always  give  fairly 
sharp  images  of  objects  at  a  reasonable  distance  from 
the  camera,  and  answer  admirably  for  beginners  and 
those  who  do  not  care  to  give  much  thought  to  their 
photography.  They  are  excellent  for  vacation  pic- 
tures, and  for  those  who  desire  to  make  mementoes 
from  time  to  time  without  becoming  familiar  with  the 
difficult  manipulation  of  a  better  lens,  and  as  a 
matter  of  fact,  the  quality  of  achromatic  meniscus 
lenses  sold  in  the  United  States  is  so  high  that  many 
pictures  made  with  such  lenses  compare  favorably 
with  those  made  with  much  more  expensive  lenses. 
It  is  an  ordinary  experience  for  an  amateur  who  has 
been  taking  pictures  with  such  a  camera  to  turn  out 
much  less  successful  work  when  he  begins  to  use  a 
focusing  camera  with  a  better  lens.  This,  however,  is 
merely  due  to  his  inexperience  with  the  more  compli- 
cated apparatus,  and  as  soon  as  he  has  learned  to 
manipulate  it  properly,  he  is  sure  to  get  better  results 
from  the  higher-grade  outfit. 

Defects  of  Meniscus  Lenses.  —  The  meniscus  lens 
usually  has  only  two  aberrations  even  partly  corrected. 
There  are  three  other  important  aberrations  of  lenses, 
and  these  are  not  corrected  in  the  meniscus  lens. 
The  meniscus  cannot  render  straight  lines  as  such 
when  they  are  near  the  margin  of  the  picture,  and  is 


16  HOW  TO  CHOOSE 

consequently  not  suitable  for  taking  pictures  of  build- 
ings, though  in  landscape  work  this  fault  is  unim- 
portant. This  defect  is  called  curvilinear  distortion. 
The  meniscus  lens  also  suffers  seriously  from  curva- 
ture of  field  and  astigmatism,  two  defects  which  will 
be  more  fully  explained  further  on,  and  the  cheaper 
grades  will  have  some  spherical  aberration  not  cor- 
rected. All  of  these  shortcomings  render  it  impossible 
for  such  a  lens  to  render  the  finest  detail  absolutely 
sharp  even  in  the  centre  of  the  field,  and  this  is  even 
more  the  case  at  the  corners.  Finally,  in  order  to 
make  it  work  at  all,  it  has  to  be  stopped  down  to  such 
a  small  aperture  that  the  lens  will  not  allow  short 
exposures  to  be  made,  and  consequently  is  useless 
under  many  circumstances. 

Meniscus  or  Rapid  Rectilinear?  —  Ordinarily  the 
choice  of  a  first  lens  for  the  beginner  consists  of  de- 
ciding between  two  cameras  of  the  same  model,  one 
fitted  with  a  single  meniscus  achromatic  and  the 
other  with  a  rapid  rectilinear  lens,  a  type  which  we 
will  fully  describe  a  little  further  on.  The  difference 
in  price  is  usually  from  two  to  six  dollars.  There  are 
a  few  cameras,  the  smaller  and  cheaper  sorts  more 
particularly,  which  are  offered  with  meniscus  lenses 
only.  Such  instruments  are  supplied  with  time  and 
instantaneous  shutters  and  are  of  the  fixed-focus  type. 
Naturally  as  the  price  is  lower  and  the  user  does  not 
need  to  bother  with  focusing,  these  simple  cameras 
have  been  sold  by  thousands  where  the  more  elaborate 
ones  have  been  disposed  of  in  hundreds.  It  is  evident 
that  they  must  give  good  results,  or  they  would  not  be 
so  popular.  Let  us,  then,  run  over  the  advantages  of 
the  meniscus  achromatic  lens  and  see  what  its  field  is. 


AND  USE  A  LENS.  17 

The  meniscus  lens  is  seldom  used  for  picture  sizes 
larger  than  4X5  or  3i  X  5l  (postcard)  and  the 
focal  length  or  distance  from  the  film  to  the  lens  is 
rarely  more  than  6j  inches.  The  stop  or  opening 
through  which  the  light  passes  is  seldom  any  larger 
than  /:i6,  which,  for  a  6j-inch  lens,  is  6|  4-  16,  or 
i%  inch.  Such  a  stop  is  only  of  medium  size, 
compared  to  the  focus  of  the  lens,  and  it  has  the  ad- 
vantage that  it  allows  a  good  average  focus  to  be 
obtained.  The  maker  carefully  adjusts  the  camera 
until  it  makes  a  reasonably  sharp  image  of  all  objects 
from  8  feet  to  the  far  distance,  and  then  arranges  the 
instrument  so  that  when  it  is  ready  for  use  the  lens 
will  always  be  at  the  right  position  to  make  this 
average  focus  sure.  A  few  postcard-size  cameras 
with  meniscus  lenses  are,  however,  provided  with  an 
adjustable  front  with  pointer  and  focusing  scale,  so 
that  the  operator  can  focus.  Distances  in  feet  are 
marked  on  the  scale,  for  example,  50,  15,  and  6.  It 
is  not  necessary  to  have  a  focusing  scale  for  the  smaller 
picture  sizes,  such  as  2j  X  2\  or  2\  X  3i,  because  the 
lenses  of  shorter  focal  length  used  for  them  have 
more  depth  of  field  and  can,  in  accordance  with 
optical  laws,  focus  at  once  objects  near  and  far. 

Finding  the  Hyperfocal  Distance.  —  The  formula 
by  which  the  depth  of  sharp  field  can  be  found  is: 
Multiply  the  square  of  the  focal  length  in  inches  by 
100  and  divide  by  the  /  number  of  the  stop.  For 
a  4-inch  lens  working  at /:8  this  would  be 

4  X  4  X  100 

=  200  inches  =  16  feet  8  inches. 

8 

The  distance  found  by  this   formula  is  called   the 
hyperfocal  distance  for  that  particular  stop.     If  the 


1 8  HOW  TO   CHOOSE 

lens  is  focused  (by  trial)  on  an  object  at  this  distance, 
it  will  be  found  that  the  depth  of  sharp  field  is  from 
half  the  hyperfocal  distance  to  the  limit  of  vision. 
Thus,  in  the  example  given,  the  4-inch  lens  will  re- 
produce all  objects  more  than  8  feet  4  inches  from  the 
camera  with  reasonable  distinctness.  If  the  smaller 
stop  of /:i6,  which  is  the  largest  usually  supplied  with 
meniscus  lenses,  is  used,  the  hyperfocal  distance  will 
become  8  feet  2  inches,  and  the  nearest  object  in 
focus  will  be  4  feet  I  inch  away.  Camera  makers, 
however,  usually  set  the  lens  so  that  it  will  not  focus 
objects  closer  than  6  feet,  in  order  to  get  a  better 
distribution  of  the  sharpness  of  definition  at  the  fur- 
ther distances.  Any  user  of  a  camera  with  a  meniscus 
lens  can  find  its  focal  length  closely  enough  for  prac- 
tical purposes,  if  the  catalog  does  not  state  the  focal 
length  in  describing  the  camera,  by  measuring  from 
the  plane  of  the  film  to  the  optical  centre  of  the 
lens,  which,  with  sufficient  accuracy  for  most  pur- 
poses, may  be  assumed  to  be  at  the  front  surface. 

Stops.  —  Fixed-focus  cameras  costing  more  than 
a  dollar  are  generally  provided  with  three  stops, 
which  take  the  form  of  holes  pierced  in  a  metal 
plate  which  can  be  moved  so  as  to  bring  any  chosen 
aperture  into  place  in  front  of  the  centre  of  the  lens. 
Each  smaller  opening  usually  has  half  the  area  of  the 
next  larger,  and  therefore  admits  only  half  as  much 
light  during  an  exposure  of  the  same  length.  When 
the  speed  of  the  shutter  is  unalterable  (that  of  most 
snap-shot  shutters,  if  unmarked,  may  be  taken  as 
3s  second),  the  only  way  one  can  regulate  the  amount 
of  light  reaching  the  film  is  by  changing  the  stop. 
If  the  largest  stop  is  /:i6,  —  and  very  few  cameras 


AND  USE  A  LENS.  19 

with  meniscus  lenses  are  furnished  with  any  larger 
aperture  —  the  other  stops  are  usually  f:22  and  f:^2 
and  require  respectively  twice  and  four  times  the  ex- 
posure of  /:i6.  They  can  be  used  for  snapshots,  in 
the  latitudes  of  the  United  States,  only  for  subjects 
of  exceptional  brilliancy,  such  as  snow  scenes  or  sea 
views,  because  exposures  on  average  landscapes  re- 
quire all  the  light  which /:i6  will  admit  when  the  sun 
is  shining  between  the  hours  of  9  A.M.  and  3  P.M. 

Shutter  Speeds.  —  The  regulation  of  exposure  by 
changing  the  size  of  the  stop  or  diaphragm  is  sup- 
plemented by  altering  the  time  the  shutter  is  allowed 
to  remain  open.  The  lowest  priced  cameras  with 
shutters  have  a  rotary  shutter  arranged  for  snapshots 
(or  "  instantaneous  ")  and  "  time;"  the  more  elaborate 
possess  one  giving  "  time,"  "  bulb,"  and  "  instan- 
taneous;" still  more  expensive  ones  provide  speeds 
marked  in  fractions  of  a  second  from  I  second  to  yitf 
second,  or  even  up  to  vfa  or  ^J^  second,  if  one  is  will- 
ing to  pay  the  price.  However,  one  would  find  a 
fast  shutter  of  little  use  with  a  meniscus  lens,  so 
manufacturers  furnish  only  the  TIB  sort  with  this 
lens.  The  "  bulb  "  exposure  is  arranged  so  that  the 
shutter  will  remain  open  as  long  as  pressure  is  main- 
tained on  the  rubber  bulb  or  on  the  flexible  metal 
release  which  has  now  almost  driven  the  bulb  and 
tube  from  the  market.  The  "  time  "  exposure  is 
arranged  so  that  it  takes  one  pressure  to  open  the 
shutter  and  a  second  pressure  to  close  it.  A  mo- 
ment's thought  will  show  that  by  combining  different 
lengths  of  exposure  with  different  stops,  one  can 
secure  almost  any  desired  amount  of  light  action 
greater  than  that  represented  by  ?V  second  at  /:i6, 


20  HOW  TO  CHOOSE 

but  nothing  less  than  A-  second  at  /:32.  The  light 
action  of  the  latter  exposure  is  too  short  for  practical 
instantaneous  use  with  a  meniscus  lens,  except  occa- 
sionally in  full  sunlight  on  snow  or  sea. 

What  Meniscus  Lenses  Will  Do.  —  The  greater 
portion  of  the  pictures  interesting  to  the  amateur  in 
the  earlier  stages  of  his  photographic  career  can  be 
taken  successfully  with  a  meniscus  achromatic  lens. 
The  beginner  is  usually  most  interested  in  photo- 
graphing friends  or  scenes  incident  to  vacations  or 
holidays.  Such  pictures  are  generally  made  in  the 
summer  time,  between  9  A.M.  and  3  P.M.,  well 
within  the  range  for  summer  snapshots  already  indi- 
cated. In  weaker  sunlight,  good  results  can  be  suc- 
cessfully obtained  by  using  a  tripod  and  giving  a 
quick  bulb  exposure  with  the  smallest  stop.  In  fact, 
if  the  tripod  is  utilized  as  much  as  possible,  there  are 
few  stationary  subjects  which  cannot  be  well  taken 
with  a  meniscus  lens.  The  smaller  the  stop,  the  more 
clear  and  brilliant  the  rendering  of  fine  detail  in  the 
picture.  The  smaller  the  stop,  the  longer  the  ex- 
posure. These  two  statements  mark  the  limitations 
of  the  lens,  and  show  that  its  work  is  practically  con- 
fined to  still  subjects.  If  the  subject  is  not  in  motion, 
it  can  usually  be  successfully  taken  with  a  meniscus 
lens,  as  far  as  the  single  factor  of  exposure  is  con- 
cerned. There  are  optical  defects  inherent  in  the 
meniscus  lens,  however,  which  render  it  unsatis- 
factory for  use  on  special  classes  of  subjects.  The 
subject  limitations  of  the  meniscus  lens  will  be  more 
apparent  after  we  have  considered  the 

Optical  Defects  of  Single  Lenses.  —  A  single  men- 
iscus achromatic  lens  cannot  be  so  made  that  it  will 


AND  USE  A  LENS.  21 

render  straight  lines  as  such  at  the  margin  of  the 
picture,  or  render  fine  detail  critically  sharp  all  over 
the  film,  or  represent  a  flat  surface  as  equally  sharp 
all  over.  For  general  amateur  photography  these 
defects  matter  only  in  architectural  subjects.  The 
single  lens  is  not  suitable  for  taking  pictures  of  build- 
ings. It  is  too  slow  for  making  snapshots  under  poor 
conditions  of  lighting.  It  cannot,  in  other  words,  be 
expected  to  do  the  same  work  as  an  expensive  lens. 
Low  price,  in  lenses,  means  small  aperture  or  work- 
ing speed,  with  consequent  limitation  to  stationary 
objects  and  tripod  exposures  if  it  is  to  be  used  on 
subjects  covering  the  whole  field  of  photography. 

Special  Types  of  Meniscus  Lenses.  —  For  artistic 
photography,  some  workers  prefer  a  lens  which  will 
not  give  a  perfectly  sharp  image,  and  so  opticians 
have  produced  the  so-called  "  soft-focus  "  meniscus 
lenses  to  satisfy  pictorial  workers.  The  original  lens 
of  this  sort,  the  "  Smith  Semi- Achromatic,"  was  in- 
tended to  give  the  same  quality  of  soft  definition  as 
the  pinhole,  but  to  possess  a  speed  of  /:6,  thus  allow- 
ing snapshot  exposures  instead  of  the  prolonged  time 
exposures  of  a  minute  or  more  needed  for  a  pinhole. 
These  lenses  are  partly  achromatic,  that  is,  they  are 
constructed  to  bring  the  yellow  visual  rays  and  the 
blue  photographic  rays  to  nearly  the  same  indefinite 
focus,  though  they  give  less  halo  or  flare  around  the 
margins  of  light-colored  masses  if  used  with  a  yellow 
ray  filter.  They  do  not,  however,  require  a  correc- 
tion of  focus,  as  does  a  lens  made  with  a  single  piece 
of  glass,  but  give  on  the  plate  the  same  effect  seen  on 
the  ground  glass.  Such  lenses  naturally  cannot  be 
used  successfully  with  any  camera  which  is  not  pro- 


22  HOW  TO  CHOOSE 

vided  with  a  ground-glass  screen,  for  the  image  is  so 
soft  and  harmonious  that  it  needs  the  most  careful 
focusing.  By  employing  a  smaller  stop,  the  user  can 
secure  somewhat  greater  sharpness  of  definition,  but 
never  critically  fine  detail.  As  the  softness  depends 
largely  on  the  optical  defect  known  as  spherical 
aberration,  it  is  easier  to  secure  the  precise  effect  one 
desires  if  the  final  adjustment  of  focus  is  made  with 
the  stop  to  be  used  for  the  exposure,  for  a  lens  having 
spherical  aberration  changes  its  focus  slightly  for 
each  stop.  The  makers  of  these  lenses  now  manu- 
facture also  a  series  of  double  lenses  of  the  same  type, 
which  are  much  faster  and  can  be  made  to  give  vari- 
ous degrees  of  correction  of  chromatic  aberration  and 
spherical  aberration  by  stopping  down. 

The  Use  of  Soft-focus  Lenses.  —  It  is  somewhat 
difficult  for  one  who  has  used  only  fully  corrected 
lenses  to  pass  to  the  soft-focus  type,  and  at  once 
master  its  peculiarities.  A  lens  of  this  class  produces 
pictures  which  are  not  in  sharp  focus  but  have  a 
softness  which  tones  down  the  hard  outlines  and 
minute  details  of  objects  without  losing  the  drawing 
or  entirely  suppressing  the  details  necessary  to  render 
textures.  There  is  no  one  plane  where  the  picture  is 
sharpest.  There  is  a  region  of  focus  rather  than  one 
plane,  and  it  is  possible  to  move  the  lens  some  little 
distance  backwards  and  forwards  without  affecting  the 
definition  to  a  marked  degree,  yet  there  is  just  one 
point  for  each  stop  where  the  effect  is  most  pleasing. 
The  lighting  conditions  govern  to  some  extent  the 
stop  which  may  be  used,  for  there  is  more  halo  around 
light-colored  objects  in  a  brilliant  than  in  dull  light- 
ing. At  any  stop,  it  is  important  to  use  the  "  out- 


AND  USE  A  LENS.  23 

side  focus  "  for  firmness  of  drawing,  for  the  "  inside 
focus  "  tends  to  produce  too  indefinite  an  effect.  In 
other  words,  the  lens  should  be  moved  away  from  the 
plate  until  the  image  goes  out  of  focus  and  then  be 
brought  back  until  the  desired  rendering  of  detail  is 
obtained.  Lenses  of  the  single  series  have  from  50 
to  60  per  cent  correction,  and  therefore  show  more 
halo  around  the  lights  than  the  doublets  of  75  per 
cent  correction,  so  that  they  must  be  stopped  down 
more  to  get  rid  of  the  flare  or  used  in  a  duller  light. 

Soft-focus  lenses  are  perhaps  best  fitted  for  home 
portraiture,  but  the  full  aperture  has  to  be  handled 
carefully,  or  there  will  be  a  band  of  halo  spreading 
from  the  lights  into  the  shadows;  for  example,  along 
a  white  collar  where  it  comes  near  a  dark  suit.  This 
effect  may  be  minimized  by  using  a  double-coated 
orthochromatic  plate  with  a  light  yellow  filter  (2x)  and 
stopping  down  to  /:8.  The  doublets  give  the  same 
effect  at  about  /:5-6;  in  other  words,  they  are  prac- 
tically twice  as  fast.  As,  however,  the  softening  of 
skin  texture  obtained  by  using  a  large  opening  is 
often  desirable,  to  say  nothing  of  the  saving  in  time, 
it  is  advisable  to  use  light  backgrounds  and  light 
draperies  and  to  employ  rather  a  flat  lighting.  If 
the  contrast  between  the  lighted  side  and  the  back- 
ground is  small,  the  halo  becomes  negligible.  The 
shadow  side  should  be  so  well  lighted  that  the  color 
of  the  flesh  is  quite  visible  in  the  deepest  shadow  as 
well  as  in  the  highest  light.  Underexposure  exagger- 
ates the  flare.  The  spherical  aberration  blends  the 
finer  details  without  losing  the  form,  so  that  minor 
skin  blemishes  disappear,  and  there  is  no  need  of  re- 
touching. Direct  prints  give  the  same  impression  of 


24  HOW  TO  CHOOSE 

skin  texture  that  one  receives  in  looking  at  a  person, 
for  one  sees  the  face  as  a  whole,  not  as  multitudinous 
lines,  freckles,  moles,  pores,  etc.  If  used  with  dis- 
cretion, lenses  of  this  type  give  results  of  much 
artistic  value,  but  diffusion  of  focus  alone  cannot 
make  a  poor  composition  into  a  picture,  and  over- 
done "  fuzziness  "  is  a  blemish  rather  than  a  merit. 

Other  Soft-focus  Lenses.  —  Several  other  varieties 
of  soft-focus  lenses  were  introduced  after  the  Smith. 
The  Spencer  Port-Land  is  a  single  meniscus  lens  pos- 
sessing a  distinctive  character  of  its  own.  It  gives 
great  softness  without  losing  the  drawing,  and  works 
nominally  at  about  /:4-5,  though  few  workers  can 
utilize  its  image  at  any  stop  larger  than  about  /:5-6, 
on  account  of  its  giving  a  number  of  overlapping 
images.  Its  softness  is  different  from  that  of  any 
other  lens.  Then  there  is  the  Wollensak  Verito,  a 
doublet  intended  chiefly  for  portraiture  and  work- 
ing at  /:4.  The  best  results  with  this  lens  are  per- 
haps obtained  by  stopping  down  to  /:5-6  or  /:6.  At 
/:i6,  the  image  is  not  very  different  from  that  of  an 
ordinary  lens.  Many  workers  look  upon  this  prop- 
erty as  an  advantage,  as  the  Verito  will  allow  them 
to  make  soft  or  sharp  pictures  at  will  by  simply 
changing  the  stop.  Yet  another  make  in  considerable 
vogue  is  the  Struss,  which  has  been  used  with  much 
success  by  several  New  York  pictorial  workers  in  the 
short  time  during  which  it  has  been  on  the  market. 
The  great  advantage  of  the  soft-focus  objectives  as  a 
class  is  that  they  allow  the  individual  to  vary  the 
quality  of  the  definition  to  suit  different  subjects  and 
to  express  his  own  personality  in  the  picture. 

Curvilinear  Distortion.  —  As  we  have  mentioned 


AND  USE  A  LENS.  25 

before,  a  single  lens  cannot  render  straight  lines  as 
such  near  the  margin  of  the  picture.  This  defect  is 
partly  dependent  on  the  varying  thickness  of  the  lens 
from  edge  to  centre,  and  partly  on  the  position  of  the 
diaphragm.  If  the  stop  is  placed  in  front  of  the  lens, 
straight  lines  are  bowed  out  from  the  centre  of  the 
picture,  producing  what  is  known  as  barrel-shaped 
distortion;  when  the  stop  is  behind  the  lens,  the  lines 
bow  in,  and  cushion-shaped  distortion  is  the  result. 
Consequently  we  cannot  use  a  single  lens  for  archi- 
tectural photography,  for  copying,  or  for  any  purpose 
where  it  is  necessary  to  reproduce  straight  lines, 
especially  near  the  edge  of  the  picture.  This  defect 
has  a  very  simple  and  obvious  remedy. 

Rapid  Rectilinear  Lenses.  —  When  two  single 
meniscus  lenses  are  mounted  at  opposite  ends  of  a 
lens  tube  with  the  diaphragm  or  stop  between  them, 
the  barrel-shaped  distortion  of  the  rear  lens  is  cor- 
rected by  the  cushion-shaped  distortion  of  the  front 
lens.  The  result  is  that  straight  lines  are  rendered 
as  such,  and  the  lens  is  therefore  called  rectilinear. 
The  speed,  or  ratio  between  focal  length  and  diameter 
of  stop,  is  also  greatly  increased  by  this  arrangement, 
being  in  fact  almost  exactly  doubled.  Most  cameras 
of  moderate  price  are  fitted  with  rapid  rectilinear 
lenses  (often  abbreviated  R.R.).  The  defects  of 
spherical  aberration  and  curvature  of  the  field,  as 
well  as  astigmatism,  are  not  eliminated,  so  the  chief 
advantages  over  the  single  lens  remain  those  already 
stated,  namely,  rectilinearity  and  greater  speed. 
Further  corrections  cannot  be  made  with  only  two 
varieties  of  glass,  crown  and  flint. 

To  illustrate  the  formation  of  a  rectilinear,  let  us 


26  HOW  TO  CHOOSE 

suppose  that  a  single  lens  of  16  inches  focal  length 
and  a  speed  of  /:i6  is  being  used  on  a  5  X  7  plate. 
The  great  focal  length  is  necessary,  since  the  field  of 
a  meniscus  lens  is  considerably  curved  and  only  the 
centre  of  it  can  be  used  if  sharp  definition  is  to  be 
obtained  at  this  large  aperture.  The  diameter  of  the 
stop  is  I  inch.  If  another  lens  exactly  like  the  first 
is  mounted  in  the  front  of  the  lens-tube,  the  focus  of 
the  combined  lens  is  now  approximately  half  that  of 
either  element,  namely  8  inches,  and  for  simplicity 
of  statement  we  may  assume  that  the  i-inch  stop  now 
has  the  value  /:8.  This  number  is  found  by  divid- 
ing the  focal  length,  8,  by  the  diameter  of  the  stop,  i. 
The  speed  of  the  lens  is  now  four  times  as  great  as 
that  of  the  single  lens,  as  shown  by  the  ratio  between 
the  squares  of  the  numbers:  —  16  X  16  is  to  8  X  8  as 
256  to  64,  or  as  4  to  i.  The  reason  for  taking  the 
squares  is  that  openings  pass  light  in  the  ratio  of 
their  areas,  and  with  circular  apertures  the  areas  are 
in  proportion  to  the  squares  of  their  diameters. 
The  /  numbers,  then,  are  based  on  this  ratio  and  ex- 
press the  relation  between  focal-length  of  lens  and 
diameter  of  opening.  Any  lens  marked  /:8,  for  ex- 
ample, no  matter  what  its  focal  length,  is  practically 
as  fast  as  any  other  lens  also  marked /:8.  There  may 
be  slight  variations  due  to  difference  in  color  and 
thickness  of  the  glass,  but  they  are  unimportant  as 
compared  with  the  permissible  latitude  of  exposure. 
In  the  example  given  above,  we  have  seen  how  a 
definite  opening  of  I  inch  may  be  either  /:i6  or  /:8, 
according  to  the  focal  length  of  the  lens  or  lenses  in 
place  in  the  lens-tube. 
Utility  of  the  Rapid  Rectilinear  Lens.  —  A  camera 


AND  USE  A  LENS.  27 

fitted  with  double  or  triple  extension  of  the  bellows 
and  a  symmetrical  or  a  convertible  rectilinear  lens 
will  be  found  a  most  useful  general-purpose  instru- 
ment. The  symmetrical  rectilinear  is  composed  of 
two  halves  alike  in  construction  and  usually  of  the 
same  focal  length,  while  the  halves  of  the  convertible 
lens  are  generally  of  different  focal  lengths.  The 
complete  lens  is  used  for  all  ordinary  work.  The 
halves  of  the  lens  may  be  used  for  landscape  or 
other  work  containing  no  important  straight  lines, 
portraits,  and  distant  objects.  Lenses  of  this  char- 
acter are  offered  under  a  great  many  fancy  names  in 
camera  catalogues;  but  they  are  all  rectilinears  and 
possess  the  faults  already  enumerated  of  spherical 
aberration  at  the  margins,  curvature  of  field,  and 
astigmatism.  Readers  are  urged  to  beware  of  putting 
extra  money  into  a  camera  because  the  rapid  rectilin- 
ear lens  is  described  as  having  unusual  corrections, 
for  which  an  extra  price  is  charged.  There  is  made 
in  the  United  States  only  one  such  lens,  the  Recti- 
graphic,  of  which  this  is  true,  and  its  superiority  re- 
sults from  the  use  of  three  instead  of  two  elementary 
lenses  in  each  of  the  single  combinations.  Aside  from 
this  lens,  if  an  extra  price  is  to  be  paid,  it  is  better  to 
invest  in  one  of  the  low-priced  anastigmat  lenses. 
Rapid  rectilinear  lenses  supplied  on  foreign  cameras 
are  generally  styled  aplanats.  The  typical  rapid  rec- 
tilinear lens  works  at  /:8,  but  it  does  not  cover  its 
plate  with  critical  sharpness  over  the  whole  surface 
unless  it  is  stopped  down  to  at  least  /:io.  Rapid 
rectilinear  lenses  are  occasionally  diaphragmed  so  as 
to  work  at  larger  apertures  than  /:8.  They  can  be 
thus  used  for  special  work,  such  as  portraiture,  but 


28  HOW  TO  CHOOSE 

cannot  give  good  definition,  as  the  larger  opening 
introduces  more  errors. 

Choosing  a  Rapid  Rectilinear  Lens.  —  When  one  is 
buying  a  camera  and  it  is  offered  in  two  models  at  a 
slight  difference  in  cost,  the  lower-priced  being  fitted 
with  a  meniscus,  and  the  higher-priced  with  a  rapid 
rectilinear,  the  choice  depends  on  whether  one  is  con- 
tent to  accept  the  disadvantages  of  the  meniscus  for 
the  sake  of  the  average-focus  feature.  If  not,  the 
rapid  rectilinear  should  unhesitatingly  be  chosen.  A 
good  rectilinear  is  an  excellent  general-purpose  lens 
for  amateur  photography.  The  makers  can  be  relied 
upon  when  one  is  buying  a  complete  camera,  whether 
film  or  plate.  The  case  is  a  little  different  when  a 
lens  is  to  be  chosen  from  several  listed  with  a  par- 
ticular camera. 

Cameras  fitted  with  a  single-extension  bellows,  such 
as  most  roll-film  and  many  film-pack  and  plate  cam- 
eras, gain  nothing  by  having  a  convertible  lens.  The 
symmetrical  form  of  rapid  rectilinear  is  usable  on 
such  instruments  only  as  a  whole.  When  a  double 
extension  of  bellows  is  provided  the  rear  lens  alone 
can  be  used.  The  more  complete  instruments  of 
folding  or  view  style,  with  triple  extension  of  bellows, 
should  preferably  be  fitted  with  a  convertible  lens. 
For  instance,  in  the  5X7  size  a  typical  outfit  has  a 
focus  of  7J  inches  for  the  doublet,  12  inches  for  the 
rear,  and  18  inches  for  the  front  element.  The  bel- 
lows extension  is  20  inches. 

Value  of  the  Rapid  Rectilinear  Lens.  —  For  general 
amateur  work,  a  camera  fitted  with  a  rapid  rectilinear 
lens  having  a  focal  length  about  equal  to  the  diagonal 
of  the  film  or  plate  is  very  suitable.  It  has  sufficient 


AND  USE  A  LENS.  29 

speed  for  snapshots  in  sunlight  at  all  seasons  of  the 
year,  and  at  reasonably  early  and  late  hours.  When 
it  is  fitted  to  a  film  camera  the  focusing  scale  has,  of 
course,  to  be  relied  on ;  and  this  may  at  first  trouble 
the  beginner.  A  few  suggestions  about  focusing  may 
be  of  assistance  here.  The  common  amateur  sizes 
are  3J  X  5i  (postcard)  and  4X5  and  smaller,  as  far 
as  hand  camera  snapshots  are  concerned.  The  focal 
lengths  of  the  lenses  usually  fitted  to  these  sizes  by 
makers  are  6f  and  6j  inches.  These  lenses  will  give 
a  good  general  focus  when  set  at  25  feet,  with  stop 
/:8  in  use.  When  set  at  15  feet  and  stopped  down  to 
/:i6,  they  focus  everything  8  feet  or  farther  from  the 
camera.  The  only  real  difficulty  in  focusing  comes 
when  objects  rather  close,  such  as  figure-studies  or 
portraits  outdoors,  are  to  be  attempted,  and  ordi- 
narily one  can  pace  off  the  distance,  or  otherwise 
measure  it,  for  accurate  estimation  of  the  distance  is 
absolutely  essential. 

The  Shutter.  —  Of  more  importance  than  the  lens 
itself,  in  one  way,  is  the  exposure-shutter  to  which 
it  is  fitted.  The  minimum  of  efficiency  with  a  rapid 
rectilinear  lens  is  obtained  when  it  is  fitted  to  the 
TIB  shutter.  Even  then,  however,  it  is  of  vastly 
greater  efficiency  than  the  meniscus,  for  the  simple 
reason  that  the  speed  of  the  meniscus  is  such  that, 
for  snapshot  work,  we  are  always  verging  on  the 
ragged  edge  of  underexposure,  whereas  with  the  rapid 
rectilinear,  a  snapshot  exposure  of  approximately  -£5 
second  at  /:8  gives  full  exposure  not  only  for  sum- 
mer work  in  various  lights,  but  also  on  an  increased 
field  of  subjects.  When,  however,  the  shutter  has 
time,  bulb,  and  regulated  speeds  marked  I,  |,  J,  *V, 


30  HOW  TO  CHOOSE 


slv,  and  Tiff  second,  the  rectilinear  is  capable  of  doing 
far  better  work.  If  The  American  Photography  Ex- 
posure Tables  are  consulted,  the  user  can  soon  count 
on  getting  a  good  picture  almost  every  time.  Con- 
sidering /:8  as  the  ordinary  aperture  for  snapshot 
work,  the  range  of  instantaneous  speeds  just  given 
will  cover  all  common  requirements.  In  fact,  we 
have  seen  many  satisfactory  pictures  made  with  a 
rapid  rectilinear  lens  set  in  an  even  simpler  shutter, 
having  T,  B,  and  three  speeds  supposed  to  run  from 
^V  to  T&T7  second.  We  are  of  the  opinion  that  the 
most  generally  useful  shutter  for  the  rapid  rectilinear 
lens  would  be  one  having  accurate  speeds  of  TO  >  sV>  ds 
and  jitf  second. 

Limitations  of  the  Rapid  Rectilinear  Lens.  —  Al- 
though for  most  work  the  astigmatism  and  curva- 
ture of  field  of  the  rapid  rectilinear  make  little 
difference,  there  are  some  pictures  in  which  the  poor 
marginal  definition  is  a  menace  to  success.  The 
fault,  strange  as  it  may  seem,  is  most  likely  to  occur 
when  the  worker  is  focusing  on  the  ground  glass. 
The  reason  is  that  he  tries  to  get  the  middle  of  the 
picture  critically  sharp.  This  is  easy,  for  the  lens  is 
well  adapted  to  give  a  considerable  fineness  of  defi- 
nition in  the  centre  of  its  field.  We  well  remember 
the  trouble  which  used  to  occur  with  a  6|  X  8j 
camera,  fitted  with  an  n-inch  rapid  rectilinear  lens. 
At  /:8,  focusing  near  the  centre,  the  sharpness  was 
confined  to  a  5-inch  circle  and  the  rest  of  the  plate 
was  very  blurry.  Through  the  help  of  a  friend,  it 
was  soon  learned  that  the  curvature  of  the  field 
would  have  to  be  split.  Instead  of  focusing  on  the 
centre,  we  marked  on  the  ground-glass  points  about 


AND  USE  A  LENS.  31 

midway  between  the  centre  and  the  margins,  focus- 
ing on  them  and  afterwards  stopping  down  a  little 
if  necessary.  This  method  should  invariably  be 
adopted  when  a  rapid  rectilinear  is  used  on  a  camera 
which  allows  focusing  on  the  ground  glass,  for  the 
results  will  thus  be  as  good  as  the  qualities  of  the  lens 
will  permit.  The  smaller  the  stop  used,  the  less  the 
effects  of  the  still  uncorrected  errors  of  the  lens  will 
show. 

Curvature  of  Field.  —  The  rapid  rectilinear  lens, 
introduced  about  1865,  represents  practically  the 
limit  of  correction  obtainable  with  the  old  glasses 
known  as  crown  and  flint.  As  we  have  already 
stated,  it  is  still  afflicted  with  serious  optical  defects, 
absolutely  irremovable  with  these  materials.  Obvi- 
ously, as  the  glass  dry  plate  must  be  flat  to  be  pre- 
pared on  any  commercially  possible  scale,  the  image 
produced  by  the  lens  should  be  in  sharp  focus  over  the 
whole  of  a  large  flat  surface.  This  is  distinctly  not 
the  case  with  the  simple  lens,  the  achromatic  com- 
bination, or  the  rapid  rectilinear.  With  each  of  these 
forms  of  lens,  the  image  is  brought  to  a  focus  on  a 
saucer-shaped  field  which,  of  course,  cannot  coincide 
with  a  flat  glass  plate,  though  the  use  of  a  small  stop 
to  a  large  degree  obviates  this  defect,  together  with 
many  others.  In  the  modern  anastigmat  this  defect 
is  almost  entirely  overcome. 

Astigmatism.  —  The  fifth  and  last  of  the  important 
aberrations  of  the  lens,  the  most  difficult  to  explain 
and  the  hardest  to  remove,  is  known  as  astigmatism. 
It  is  not  only  an  affliction  of  the  artificial  lens  made 
in  the  workshop,  but  is  unfortunately  known  by  name 
and  effects  to  thousands  of  wearers  of  eyeglasses,  who 


32  HOW  TO  CHOOSE 

probably  have  little  conception  of  just  how  their 
affliction  is  caused.  It  is  an  optical  defect  by  reason 
of  which  a  lens  cannot  bring  to  a  sharp  focus  at  one 
time  lines  which  run  in  different  directions  on  a 
plane  surface.  The  oculist's  test  chart  contains  a 
figure  made  of  radial  lines  looking  like  the  spokes 
of  a  wheel.  The  reader  whose  eyes  are  affected  with 
astigmatism  will  see  most  of  these  lines  sharp,  but 
one  or  more  blurred.  So  the  photographic  lens  which 
is  not  corrected  for  astigmatism  will  render  a  series 
of  lines  covering  the  whole  surface  of  the  plate  sharp 
in  some  portions  of  the  image,  and  blurred  in  others. 
By  racking  the  lens  back  and  forth  any  given  line 
can  be  made  sharp,  but  some  others,  running  in  a 
different  direction  on  the  same  part  of  the  plate,  or 
in  the  same  direction  on  a  different  portion  of  the 
plate,  will  be  blurred.  The  defect  is  due  to  the  in- 
ability of  the  marginal  portions  of  a  lens  made  of 
ordinary  crown  and  flint  glass  to  bring  to  a  focus  in 
the  same  plane  the  images  of  lines  radial  to  the  lens 
and  tangential  to  it.  When  one  is  sharp,  the  other 
will  be  blurred. 

Mathematicians  proved  that  astigmatism  could  be 
removed  from  lenses  by  the  production  of  glass  of 
certain  definite  qualities  and  a  German  chemist  had 
faith  that  such  glasses  could  be  produced.  The 
chemist,  Schott,  and  the  scientist,  Abbe,  joined  hands 
in  1884  and  founded  the  famous  glass-works  at  Jena, 
which,  with  financial  aid  from  the  Prussian  govern- 
ment, finally  produced  not  only  the  single  sort  of 
dense  barium  crown  glass  of  high  refractive  index 
which  was  necessary  for  the  first  anastigmat,  but  an 
enormous  series  of  new  types  of  glass  of  the  most 


AND  USE  A  LENS.  33 

varied  properties,  which  meets  almost  every  need 
of  the  optician,  and  has  allowed  the  production  of 
lenses  of  qualities  quite  undreamed  of  a  generation 
ago.  In  curing  astigmatism,  the  optician  has  also 
been  able  to  mold  the  other  qualities  of  the  lens  al- 
most at  his  will. 

Advantages  of  the  Anastigmat.  —  Most  anastig- 
mats  are  not  only  free  from  astigmatism  but  possess 
also  very  complete  corrections  for  chromatic  aber- 
ration, spherical  aberrations  of  different  classes,  and 
curvature  of  the  field.  As  most  of  these  faults  are 
well  removed  in  the  rapid  rectilinear  lens  at  a  work- 
ing aperture  of /:i6  or  smaller,  the  chief  advantage  of 
the  anastigmats  is  speed.  They  do  not  require  stop- 
ping down  to  secure  crisp,  brilliant  definition  from 
corner  to  corner  of  the  plate.  They  do  not  need  any 
special  precautions  in  focusing  the  image  of  a  flat 
object,  for  the  image  is  almost  as  flat  as  the  plate, 
instead  of  being  saucer-shaped,  as  with  the  simpler 
lenses.  They  require,  it  is  true,  somewhat  more  care 
than  the  R.  R.  in  focusing.  A  very  slight  move- 
ment of  the  focusing  screw  is  sufficient  to  produce  all 
the  difference  between  definition  no  better  than  that 
of  a  common  lens  and  the  most  beautiful,  minute 
sharpness.  The  margins  of  the  picture  are  just  as 
crisp  and  clear  as  the  middle,  even  if  the  sharpest  focus 
is  secured  at  the  centre.  In  other  words,  the  anas- 
tigmat  at  full  aperture  gives  a  sharper  picture  than 
the  R.  R.  will  give  unless  the  latter  is  stopped  down 
to  /:i6  or  smaller.  This  means  that  the  practical, 
everyday,  available  speed  of  the  anastigmat  is  from 
4  to  7  times  that  of  the  rapid  rectilinear,  depending  on 
the  full  aperture  of  the  former.  Even  the  lower-priced 


34  HOW  TO  CHOOSE 

anastigmats  working  at  /:8  or  /:77  show  this  real, 
practical  increase  of  speed  when  compared  with  rapid 
rectilinears  working  at  the  same  nominal  aperture. 
The  anastigmat  need  never  be  stopped  down  save  to 
secure  increased  depth  of  sharp  field.  This  brings  us 
to  a  consideration  of  the  next  step  in  the  price  of  an 
outfit  furnished  with  a  choice  of  rapid  rectilinear  and 
anastigmat  lenses. 

Rapid  Rectilinear  or  Anastigmat.  —  Reference  to 
almost  any  catalogue  of  cameras  will  show  that  an 
anastigmat  working  at/:8  or/:7.7  is  supplied  for  from 
$5  to  $10  more  than  the  rapid  rectilinear  lens  equip- 
ment. More  can  be  paid  if  one  wishes  a  shutter 
with  a  wider  range  of  speeds  or  a  lens  working  at 
/:6.8,  /:6.3,  or  faster.  However  as  the  vast  bulk  of 
amateur  work  is  done  with  apertures  of /:8  or  smaller, 
and  we  may  consider  /:8  the  average  aperture  for 
general  snapshot  work,  it  is  worth  while  appraising 
the  slower  anastigmats.  Should  a  particular  camera 
be  desired,  if  the  makers  do^not  offer  this  type  of 
anastigmat  lens,  the  apparatus  may  be  bought  fitted 
with  the  rapid  rectilinear,  and  the  anastigmat  lens  in 
cells  may  be  purchased  separately  and  screwed  into  the 
shutter  in  place  of  the  glasses  of  the  rapid  rectilinear. 
One  American  lens  maker  offers  a  set  ready  for  sub- 
stitution in  the  shutter  of  any  3^  X  5?  kodak  or 
camera  for  $10.  The  same  maker  offers  an  /:6.8  lens 
and  shutter  for  $20  in  postcard  size.  Several  other 
manufacturers  offer  similar  opportunities  to  fit  rapid 
rectilinear  outfits  with  anastigmat  equipment.  The 
reader  can  easily  see  that  price  need  no  longer  be  a 
serious  obstacle  to  his  owning  a  high-grade  lens. 

Price,  notwithstanding,  remains  an  important  con- 


AND  USE  A  LENS.  35 

sideration.  The  novice  finds  it  difficult  to  under- 
stand why  some  anastigmats  can  be  furnished  with 
a  camera  for  only  $5  increase  in  price  while  others 
cost  $20  to  $50  more.  There  are  several  reasons  for 
the  great  range  of  prices.  Manufacturing  cost  is  the 
most  important.  A  lens  with  simple  construction 
and  shallow  curves  can  be  ground  and  polished  in  a 
very  small  fraction  of  the  time  required  for  deeper 
curves.  The  cost  of  mounting  and  testing  varies 
tremendously  for  different  types  of  construction.  In 
every  instance,  the  faster  the  lens,  the  more  difficult 
it  is  to  secure  perfection,  so  the  higher  must  be  the 
price.  The  construction  is  usually  more  complicated, 
because  it  is  difficult  to  correct  a  large  aperture  with 
a  simple  form  of  construction.  These  causes  may 
make  an  /:4-5  lens,  for  instance,  cost  more  to  manu- 
facture than  one  of  a  similar  type  working  at  /:6.3. 
Speed,  that  desirable  quality  which  allows  very  short 
exposures,  must  be  paid  for. 

Some  formulas  allow  much  more  complete  cor- 
rection of  certain  optical  errors  than  other  apparently 
similar  formulas.  The  older  anastigmats  generally 
retained  some  spherical  aberration,  particularly  the 
variety  known  as  coma,  which  slightly  softened  the 
image.  Getting  rid  of  coma  in  the  latest  types  has 
been  a  long  and  costly  struggle,  but  most  makers 
have  now  produced  lenses  which  are  as  finely  cor- 
rected as  it  is  possible  for  objectives  to  be.  In  fact, 
it  is  doubtful  whether  some  of  the  present  lenses  can 
be  surpassed,  unless  some  method  of  economically 
grinding  curved  surfaces  more  complex  than  those  of 
the  sphere  can  be  devised. 

Although  anastigmats  vary  in  excellence,  all  are 


36  HOW  TO  CHOOSE 

better  than  rapid  rectilinears  and  few  workers  are 
likely  to  demand  of  even  the  cheaper  ones  more  than 
they  are  capable  of  doing.  It  is  only  when  one  is 
doing  special  work,  such  as  copying  maps  or  plans, 
etc.,  that  a  minute  amount  of  coma  would  interfere 
with  the  result.  In  general  amateur  and  professional 
photography,  the  anastigmat  will  answer  all  purposes 
even  if  it  is  not  perfectly  corrected.  To  choose  one 
is  a  problem  which  can  be  solved  only  by  knowing 
with  what  apparatus  the  lens  is  to  be  used  and  what 
classes  of  work  are  to  be  attempted.  We  advise 
using  some  anastigmat,  if  possible;  but  the  exact  sort 
to  get  is  something  which  each  user  must  decide  for 
himself. 

The  Types  of  Anastigmats.  —  In  the  course  of 
time,  partly  to  enable  new  manufacturers  to  produce 
lenses  which  should  not  conflict  with  previous  patents, 
and  partly  because  opticians  have  ceaselessly  tried 
to  improve  upon  prior  achievements,  the  number  of 
types  of  anastigmats  has  steadily  increased.  It  may 
be  useful  for  us  to  review  some  of  these,  especially 
those  which  have  been  placed  upon  the  American 
market,  for  there  are  a  number  of  anastigmats  which 
are  not  sold  in  this  country. 

In  the  first  place,  it  may  be  of  interest  to  tell  just 
how  the  first  anastigmat  became  possible.  Before 
the  Jena  glass  factory  was  founded,  there  were  but 
two  kinds  of  optical  glass,  crown  and  flint,  and  from 
this  normal  glass-pair,  as  it  is  called,  opticians  were 
able  to  make  only  a  chromatically  corrected  lens 
without  astigmatic  correction.  The  collecting  lens  in 
this  type  both  refracts  and  disperses  less  than  the 
dispersing  lens.  To  get  the  anastigmatic  correction, 


AND  USE  A  LENS.  37 

it  was  necessary  to  make  the  collecting  lens  of  a  more 
highly  refractive  glass  than  the  dispersing  lens,  while 
the  ratio  of  the  dispersions  was  similar  to  that  of  the 
older  glasses.  Abbe  and  Schott  made  such  glasses, 
and  the  combination  is  called  an  anomalous  glass- 
pair.  The  achromatic  lenses  made  from  these  are 
called  new  achromats  to  distinguish  them  from  the 
old  achromats.  The  new  achromat  can  be  corrected 
or  over-corrected  for  astigmatism,  but  not  for  spheri- 
cal aberration,  while  the  case  is  reversed  with  the  old 
achromat.  The  combination  of  an  old  achromat  and 
a  new  achromat  allows  the  correction  of  both  errors, 
as  well  as  distortion,  and  the  remaining  aberration, 
curvature  of  field,  is  disposed  of  by  properly  separat- 
ing the  two  groups  of  lenses. 

The  Protar.  —  The  first  anastigmat  was  made  on 
this  basis  in  1890,  and  was  calculated  by  Rudolph  of 
Jena  for  the  Zeiss  works.  It  was  an  unsymmetrical 
doublet,  with  central  stop,  and  is  still  marketed  as 
the  Series  V  Protar,  used  for  wide-angle  work.  The 
front  combination  is  the  same  as  that  of  a  rapid 
rectilinear  lens,  while  the  rear  half  is  a  new  achromat, 
the  fourth  glass  being  of  the  heaviest  barite  crown, 
the  glass  which  makes  anastigmats  possible. 

Anastigmatic^Correction  of  the  Single  Lens.  —  The 
next  step  in  the  progress  of  the  anastigmat  was  the 
production  of  a  single  lens,  which  was  achieved  by  the 
combination  of  three  glasses  in  the  form  of  half  of  a 
rapid  rectilinear  in  which  the  rear  glass  has  been 
replaced  by  a  new  achromat.  Rudolph  immediately 
developed  this  further  into  a  four-glass  combination, 
which  was  composed  of  a  new  achromat  and  an  old 
achromat  cemented  together.  This  lens,  developed 


38  HOW  TO  CHOOSE 

in  1895,  is  the  basis  of  the  Series  VII  Protars,  working 
at  / 112.5.  The  corrections  for  the  various  aber- 
rations made  possible  by  this  formula  are  very  satis- 
factory, so  that  these  single  lenses  are  excellent 
anastigmats  of  great  brilliancy  and  crispness  of  defi- 
nition. They  are  so  slow  that  they  find  little  favor 
today  except  in  the  form  of  sets. 

Convertible  Sets.  — The  Series  VII  Protar  is  the 
basis  of  the  convertible  Protar  sets.  Each  set  con- 
sists of  a  number  of  similar  objectives  with  front 
stops,  which  may  be  screwed  into  the  two  ends  of  the 
lens  tube,  so  that  they  can  be  used  either  singly  or  in 
pairs.  A  set  of  two  elements  of  different  focal  lengths 
gives  three  focal  lengths,  three  elements  give  six 
focal  lengths,  and  four  elements  give  ten  focal  lengths. 
Such  a  combination  of  like  or  similar  halves  auto- 
matically corrects  distortion,  coma,  and  some  other 
lens  errors.  The  double  objective  is  therefore  better 
corrected  than  the  single  lens,  and  has  a  much  larger 
aperture.  With  such  sets  it  is  possible  to  photograph 
a  given  object  from  any  point  in  different  sizes. 
In  combining  the  single  lenses,  it  is  advisable  not 
to  exceed  the  ratio  of  two  to  three  in  focal  length  be- 
tween the  elements,  the  [lens  of  greater  focal  length 
being  always  placed  in  front  of  the  diaphragm.  When 
the  single  element  is  used,  it  should  always  be  placed 
in  the  rear  end  of  the  tube  with  the  stop  in  front. 
When  two  Series  VII  Protars  are  permanently  com- 
bined, we  get  a  doublet  known  as  the  Series  Vila, 
working  at  the  aperture  of  /:6.3  instead  of  the/:i2.5 
of  the  single  lens. 

The  Protar  has  never  been  surpassed  as  a  sym- 
metrical anastigmat  doublet,  with  the  possibility  of 


AND  USE  A  LENS.  39 

using  the  halves  singly.  Its  nearest  competitors  in 
this  respect  are  the  three-glass  cemented  elements  of 
the  Dagor  and  Collinear  types.  The  halves  of  a  large 
number  of  symmetrical  anastigmats,  especially  those 
having  elements  composed  of  two  separate  lenses, 
cannot  be  used  singly,  because  they  would  have  to 
be  stopped  down  too  sharply  in  order  to  obtain  a 
moderately  useful  image. 

The  Double  Anastigmat.  —  While  Rudolph  was 
working  out  his  Protars,  Von  Hoegh  of  the  Goerz 
firm  elaborated  the  three-glass  element  differently, 
by  combining  two  identical  elements  of  Rudolph's 
first  type  to  form  the  double  anastigmat,  now  known 
as  the  Dagor.  In  his  opinion,  his  doublet  lens  was 
better  corrected  than  Rudolph's  Protar,  but  the 
elements  of  the  Dagor  are  not  as  well  corrected  as 
those  of  the  Protar.  The  effective  aperture  is  /:6.8 
for  the  shorter  foci  and  less  for  the  longer.  This  lens 
is  probably  the  most  successful  anastigmat  ever 
constructed.  It  has  been  made  by  hundreds  of  thou- 
sands, and  as  the  patents  have  expired,  it  is  now  con- 
structed under  various  names  by  practically  all  lens 
makers.  Its  maximum  stop  value  of  /:6.8  is  nomi- 
nally 50  per  cent  faster  than  that  of  the  rapid  rectilin- 
ear lens.  Actually  it  is  at  least  five  times  as  fast  as 
this  lens,  for  the  rapid  rectilinear  must  be  stopped  to 
/:i6  to  approach  it  in  defining  power.  At  its  full 
working  aperture  the  Dagor  has  almost  perfect  flat- 
ness of  field,  with  critical  definition  to  the  extreme 
corners  of  the  plate.  Half  of  the  lens  is  a  good  land- 
scape lens.  The  complete  doublet  used  with  medium 
or  small  stops  is  almost  free  from  optical  errors  over 
its  entire  circle  of  illumination,  and  therefore  can  be 


40  HOW  TO  CHOOSE 

used  on  plates  larger  than  those  for  which  it  is  listed, 
as  a  wide-angle  lens. 

Other  Three-glass  Cemented  Systems.  —  Other 
anastigmat  elements  have  been  used  by  various 
makers.  One,  calculated  by  Von  Hoegh,  has  long 
been  made  by  Watson  of  London,  under  the  name 
Convertible  Lens,  while  the  Zeiss  Convertible  Series  IV 
is  of  the  same  type.  Another  invention  of  this  type 
was  produced  simultaneously  by  Voigtlander  of 
Brunswick  and  Steinheil  of  Munich,  and  introduced 
under  the  respective  names  of  Collinear  and  Ortho- 
stigmat.  These  lenses  not  only  work  at  /77  to  /:6.8, 
but  can  also  be  satisfactorily  corrected  in  smaller  sizes 
to  work  at  /:54.  As  new  glasses  have  been  pro- 
duced at  Jena,  it  has  been  possible  still  further  to 
improve  this  class  of  lenses,  until  one  of  them,  the 
Euryplan  Series  Va,  works  satisfactorily  at  the  high 
speed  of  /4-5,  about  three  times  as  fast  as  the  rapid 
rectilinear  at  its  largest  opening. 

Eight-  and  Ten-lens  Double  Objectives.  —  The 
Rietzschel  Linear  is  similar  to  Rudolph's  four-glass 
element,  differing  in  the  arrangement  of  its  dis- 
persing and  collecting  surfaces.  In  practice  this  and 
other  eight-glass  lenses  do  not  surpass  the  six-glass 
symmetrical  anastigmat,  while  the  manufacturing 
difficulties,  especially  that  of  centering  the  various 
glasses  in  assembling  the  lens,  naturally  increase. 
For  this  reason  opticians  have  not  looked  with  favor 
on  increasing  the  number  of  glasses,  and  while  ten- 
lens  systems  have  been  constructed,  the  only  one  now 
on  the  market  is  the  Turner-Reich.  Increasing  the 
number  of  glasses  may  increase  the  efficiency  of  the 
single  element,  but  produces  no  improvement  in  the 


AND  USE  A  LENS.  41 

doublet  in  proportion  to  the  increased  expense  of 
production.  It  is  possible  that  there  remains  for 
this  type  of  lens  a  great  increase  of  working  aperture, 
but  this  field  has  not  yet  been  thoroughly  investigated. 

Triplets  with  Air  Spaces.  —  In  1894,  H.  Dennis 
Taylor,  scientific  director  for  the  famous  English  tele- 
scope manufacturer,  Cooke,  invented  a  lens  consist- 
ing of  three  single  glasses  separated  by  air  spaces, 
which  effectively  eliminated  coma.  This  is  now 
manufactured  in  two  types,  one  working  at  f:g  to 
/:6.8,  the  faster  series  working  at  /:5-6  to  /:4-5-  The 
Cooke  lenses,  and  similar  ones  made  by  Voigtlander 
under  the  names  Triple  Anastigmat  and  Portrait 
Anastigmat,  are  not  convertible,  but  where  this 
feature  is  not  required  it  is  hard  to  find  a  more  satis- 
factory objective,  as  the  corrections  of  the  different 
series  are  well  carried  out,  and  the  lenses  are  of  very 
compact  construction  and  light  weight,  especially 
when  mounted  in  aluminum  barrels.  As  the  Cooke 
lenses  are  completely  free  from  coma,  they  have  been 
found  of  great  value  for  photo-engraving  purposes, 
and  a  majority  of  all  lenses  used  in  the  United 
States  for  this  purpose  are  of  this  type. 

Heliar,  Tessar,  and  Dynar.  —  Other  makers  soon 
produced  types  derived  from  the  Cooke  by  the  in- 
troduction of  another  kind  of  glass  and  of  a  collecting 
cemented  surface.  The  Voigtlander  Heliar  was  cal- 
culated in  1902  by  Harting,  and  in  this  lens  the  zonal 
errors  are  so  minimized  that  the  lens  can  be  con- 
structed with  the  full  aperture  of  /:4-5,  up  to  a  focal 
length  of  24  inches.  The  Heliar  is  a  five-glass  system, 
containing  two  doublets  with  a  single  lens  between, 
all  separated  by  air  spaces.  The  lens  is  entirely  free 


42  HOW  TO  CHOOSE 

from  coma  over  the  whole  field.  The  Zeiss  Tessar, 
calculated  by  Rudolph  in  1902,  contains  four  glasses, 
the  rear  cemented  combination  consisting  of  two 
glasses.  The  first  Tessar  worked  at  /:6.3,  but  it  is 
now  made  for  general  use  to  work  at  /:4-5,  and  for 
cinematography  to  work  at  /:3-5-  Harting  states 
that  the  Tessar  and  the  Heliar  are  as  completely 
corrected  as  it  is  possible  to  make  lenses,  and  con- 
siders them  unlikely  ever  to  be  surpassed.  They 
have  no  excessive  curvatures  and  the  spherical  errors 
are  completely  removed  over  the  whole  field.  The 
ic  Tessar  covers  a  larger  area  in  proportion  to  focal 
length  than  any  other  lens  yet  made.  Harting  also 
calculated  for  Voigtlander  another  five-glass  combi- 
nation, the  Dynar,  which  works  at  /:6,  and  is  in- 
tended for  general  hand-camera  use.  It  shows  the 
same  complete  corrections  as  the  Heliar. 

Four-glass  Air-space  Types.  —  The  replacement 
of  the  central  glass  by  an  air  space  in  the  single  com- 
bination of  the  Orthostigmat  or  Collinear  type  re- 
sulted in  producing  lenses  of  somewhat  less  compli- 
cated construction,  but  of  very  good  efficiency. 
Such  lenses  have  been  produced  by  various  makers 
in  speeds  up  to  /:4-5-  Among  them  we  may  mention 
the  Ross  Homocentric,  and  the  Goerz  Celor  and 
Syntor.  These  lenses  have  good  anastigmatic  cor- 
rections. There  are  many  other  lenses  of  this  type 
on  the  market,  but  some  of  them,  while  well  cor- 
rected for  astigmatism,  show  a  great  amount  of  coma. 
For  some  purposes  this  is  no  disadvantage,  but  in 
many  cases  it  is  found  necessary  to  stop  them  down 
strongly  to  overcome  this  defect. 

Miscellaneous  Types.  —  In  addition  to  the  many 


AND  USE  A  LENS.  43 

groups  of  lenses  already  enumerated,  there  have  been 
marketed  numerous  others,  from  the  Aldis,  with  two 
cemented  glasses  in  front  and  a  single  one  behind  the 
diaphragm,  a  lens  which  gives  a  most  remarkable 
sharpness  of  definition,  to  the  air-space  lenses  contain- 
ing many  separate  glasses,  and  cemented  ones  contain- 
ing as  many  as  ten  elementary  lenses.  Some  of  these 
lenses  are  of  the  highest  quality,  while  others  are  in- 
ferior. In  buying  lenses,  one  has  to  rely  partly  on 
the  reputation  of  the  maker  and  partly  on  the  results 
of  test  exposures.  The  principal  point  of  dispute  be- 
tween lens  makers  today  is  as  to  the  relative  efficiency 
of  cemented  and  air-space  lenses. 

Why  is  there  a  Difference  Between  Anastigmats?  — 
At  this  point  the  reader  may  ask,  "  What  is  the  neces- 
sity for  so  many  types  of  anastigmats  and  which  is 
actually  the  best?"  The  only  answer  that  can  be 
given  is  that  there  is  no  best  anastigmat,  because 
with  the  number  of  variable  elements  at  the  com- 
mand of  the  lens  calculator,  it  is  not  possible  in  any 
lens  to  remove  all  the  aberrations  for  light  of  all 
wave  lengths,  and  for  all  zones  of  the  lens.  A  mathe- 
matical optician  must  choose  which  aberrations  he 
will  completely  remove  over  the  whole  field,  and  for 
all  colors,  and  calculate  his  lens  accordingly.  If  he 
is  making  a  lens  for  the  three-color  worker,  he  must 
produce  three  sharp  images  of  three  different  colors 
which  will  be  coincident  in  size,  and  lenses  of  this 
type  are  called  apochromatic.  If  he  is  making  a  lens 
for  the  process-engraver,  he  must  endeavor  to  remove 
coma  absolutely,  probably  at  the  expense  of  speed. 
For  the  hand  camera  worker,  speed  is  the  greatest 
consideration,  but  a  moderately  large  field  must  be 


44  HOW  TO  CHOOSE 

covered.  For  cinematography  everything  can  be 
sacrificed  to  the  production  of  enormous  aperture, 
even  down  to  /:i.9«  Again,  competition  in  price 
exists,  and  simplicity  of  manufacture  must  therefore 
be  taken  into  account  in  low-priced  lenses,  necessi- 
tating fewer  glasses,  fewer  cemented  surfaces  and 
shallow  curves,  even  at  some  expense  in  corrections. 

The  Performance  of  Modern  Unsymmetrical  An- 
astigmats.  —  The  performance  of  lenses  seems  to 
have  reached  its  greatest  possible  height,  under 
present  conditions,  in  lenses  containing  air-spaces, 
such  as  the  Cooke,  Heliar,  Ross,  Tessar,  and  others. 
In  these  lenses  there  is  such  a  high  degree  of  freedom 
from  aberration  with  large  aperture  and  wide  angle 
of  view,  that  no  practical  necessity  exists  for  im- 
provement. Some  may,  however,  prefer  symmetri- 
cal lenses  with  cemented  halves,  because  with  them 
we  have  the  choice  of  a  long  or  short  focus.  Such 
lenses  will  give  absolute  sharpness  at  the  edge  of  the 
plate  at  full  aperture,  and  if  stopped  down  will  give 
complete  sharpness  of  definition  over  the  whole  field 
of  illumination,  so  that  the  modern  universal  ob- 
jectives, as  we  may  name  systems  working  at  from 
/:6  to  /:7,  are  really  wide  angle  lenses  which  define 
just  as  well  as  an  astigmatic  wide  angle,  but  at  a 
much  larger  opening.  In  this  connection  we  may  re- 
mark that  if  extremely  wide  angles  are  required  the 
Hypergon  double  anastigmat  of  Goerz,  which  consists 
only  of  meniscus  lenses  with  a  diaphragm  between, 
works,  when  focused  at  /:22  and  exposed  at  /:3i, 
over  the  enormous  angle  of  140  degrees,  an  angle 
which  constructive  optics  can  never  hope  to  increase. 

The  Limit  of  Effective  Aperture.  —  While  anastig- 


AND  USE  A  LENS.  45 

mats  for  cinematographic  purposes  can  easily  be  made 
to  give  perfect  anastigmatism  at/:3.5,  and  satisfactory 
performance  down  to/: 1. 9,  for  hand  camera  work  or 
any  purpose  of  general  photography,  no  aperture 
larger  than  /4.5  is  useful  to  the  average  pho- 
tographer. At  any  larger  opening,  while  greater 
rapidity  of  exposure  can  be  obtained,  it  is  not  pos- 
sible to  obtain  sufficient  depth.  A  lens  of  this  speed 
must  be  focused  with  the  utmost  care,  and  prac- 
tically the  only  finder  which  is  satisfactory  is  the 
camera  itself.  For  some  purposes  focusing  on  a 
ground  glass  with  a  tripod  would  be  satisfactory,  but 
for  speed  work  a  lens  of  this  aperture  should  always 
be  used  on  a  reflecting  camera,  for  only  with  an  in- 
strument of  this  type  is  it  possible  easily  and  cer- 
tainly to  obtain  critical  focus.  In  general,  the  lens 
working  at  /4-5  cannot  give  as  great  an  extent  of 
sharp  field  as  one  working  at/:8,  for  even  when  a  faster 
lens  is  stopped  down  to  the  same  aperture,  it  is  sur- 
passed by  the  slower  lens,  because  in  sacrificing  speed 
it  is  possible  to  correct  astigmatism  over  a  larger 
field.  As  to  sharpness  in  the  centre  of  the  field,  both 
anastigmats  and  astigmats  may  have  good  definition, 
depending  on  the  perfection  of  the  corrections  for 
spherical  aberrations.  But  certain  types  of  anastig- 
mats, especially  the  Tessar,  Heliar,  and  Dynar,  give 
a  central  sharpness  which  is  sought  in  vain  among 
the  astigmatic  lenses.  Lenses  working  at  /:3-5  are 
used  to  a  slight  extent  by  press  photographers  and 
others  who  believe  they  must  possess  the  utmost 
possibility  of  speed. 

Air-Spaces.  —  Some    makers    who    produce    only 
cemented    lenses    intimate    that    any    air-space    lens 


46  HOW  TO  CHOOSE 

must  be  inferior  because  of  the  internal  loss  of  light 
by  reflection.  Their  opponents  retort  that  thick 
cemented  lenses  lose  as  much  light  by  absorption  as 
the  air-space  lenses  do  by  reflection.  Dr.  Hans  Hart- 
ing  says,  "  The  question  of  the  relation  of  actual 
brilliancy  in  the  case  of  cemented  and  uncemented 
lenses  of  the  same  relative  opening  is  very  often 
raised.  Absorption  depends  on  the  kind  of  glass; 
as  in  the  same  melt  various  portions  of  glass  may 
vary  with  respect  to  absorption,  the  loss  of  light  may 
vary  somewhat  even  in  two  lenses  of  the  same  con- 
struction and  focal  length.  For  several  years  I  had 
the  opportunity  of  comparing  almost  uninterruptedly 
for  their  photographic  brilliancy  a  six-lens  symmetrical 
objective,  consisting  of  two  halves  of  three  cemented 
lenses,  and  an  objective  consisting  of  three  single 
lenses.  Both  anastigmats  of  medium  focal  lengths 
have  the  same  relative  aperture,  either  /:6.8  or  /7-7. 
I  was  never  able  to  distinguish  a  photographically 
measurable  difference  in  the  actual  brilliancy  of  these 
two  kinds  of  objectives  representing  the  extremes  of 
construction.  It  is  true  that  the  loss  of  light  in  the 
simple  landscape  lens  is  less  than  in  the  Zeiss  Double 
Pro  tar  consisting  of  eight  lenses.  But  even  this 
difference  is  unimportant  in  picture  taking,  in  com- 
parison with  the  far  greater  deviations  from  the 
proper  exposure  in  sunlight  which  even  the  most  ex- 
perienced photographer  cannot  avoid,  but  which  are 
made  absolutely  imperceptible  in  the  process  of 
development."  The  truth  seems  to  be  that  well- 
designed  air-space  objectives  pass  as  much  light  as 
cemented  ones  of  the  same  effective  aperture.  Under 
unfavorable  circumstances,  too  many  air-spaces  cause 


AND  USE  A  LENS.  47 

general  fogging  of  the  image  through  scattered  light 
inside  the  camera,  but  this  fault  is  mainly  to  be 
ascribed  to  the  fact  that  the  circle  of  illumination  is 
much  larger  than  the  plate  covered,  so  that  light  is 
reflected  from  the  inside  of  the  bellows.  The  obvi- 
ous remedy  for  this  is  to  employ  a  lens  hood,  which 
will  eliminate  the  light  before  it  enters  the  camera. 
If  bright  lights  appear  in  the  picture,  as  in  night 
photography,  the  air-space  lens  may  show  false 
images  called  ghosts,  or  spots  of  light  called  flare 
spots,  but  the  only  lens  which  would  not  show  these 
under  such  trying  conditions  is  a  single  lens  contain- 
ing only  one  piece  of  glass.  These  images  caused  by 
internal  reflection  are  often  so  much  less  bright  than 
the  original  that  they  do  not  appear  in  the  photo- 
graph, though  they  may  be  visible  on  the  ground 
glass.  General  fog  ascribed  to  air-spaces  is  often 
due  to  lack  of  sufficient  sodium  sulphite  in  the 
developer,  and  in  this  case  will  disappear  if  the 
developer  is  so  compounded  that  each  fluid  ounce 
contains  ten  grains  of  anhydrous  sulphite. 

The  choice  between  cemented  and  air-space  lenses, 
then,  depends  chiefly  on  the  value  of  convertibility, 
adaptability  to  wide  angle  work,  and  smaller  factors. 
If  one  is  using  only  a  film  camera  with  single  exten- 
sion of  bellows,  the  air-space  type  of  lens  is  perfectly 
satisfactory.  If  one's  camera  has  double  or  triple 
bellows  extension,  the  symmetrical  or  convertible 
anastigmat  may  be  preferable,  as  it  enables  one  to  do 
more  kinds  of  work.  This  then  brings  us  to 

The  Choice  of  an  Anastigmat.  Speed.  —  The  first 
consideration  is  that  of  speed.  Anastigmat  lenses  are 
offered  of  all  speeds  from /:8  to/:i.9,  the  latter  for 


48  HOW  TO  CHOOSE 

motion-picture  work  only.  The  higher  speeds  are 
only  really  available  for  general  work  in  lenses  of 
moderate  focal  length,  for  the  depth  of  field  is  so 
small  in  the  larger  sizes  that  they  can  be  used  only  on 
objects  which  do  not  move  and  which  are  nearly  in 
one  plane.  This  confines  their  use  almost  wholly  to 
portraiture  and  copying.  For  general  photography, 
the  greatest  speed  ever  useful  is/4.5.  Lenses  of  this 
speed  may  be  used  very  successfully  on  hand  cameras 
in  miniature  sizes.  Above  3J  X  4i  they  should  be 
used  only  on  reflecting  cameras  or  those  provided 
with  a  focusing  screen,  and  for  general  photography 
5  X  7  is  the  useful  limit  of  size  with  this  speed. 
With  a  6|  X  8J  plate,  /:5.6  is  about  the  limit,  while 
/:6  to  /:6.3  marks  the  practical  speed  for  8  X  10. 
Of  course,  in  portraiture,  photo-engraving,  etc.,  /:4-5 
lenses  up  to  a  focal  length  of  24  inches  are  often 
used.  For  the  photography  of  rapidly  moving  ob- 
jects and  for  general  work  in  bad  lights  the  /4.5 
anastigmats  are  indispensable,  but  the  lack  of  depth 
of  field  and  consequent  difficulty  of  focusing  render 
the  use  of  the  full  aperture  inadvisable  except  in 
case  of  necessity,  and  so  lenses  working  at  /:6.3  and 
/:6.8,  usually  spoken  of  as  "  universal  "  lenses,  are  to 
be  preferred  for  general  photographic  work  where  ex- 
cessive speed  is  not  needed. 

Focus.  —  Other  things  being  equal,  the  most 
pleasing  angle  of  view  for  probably  90  per  cent  of  all 
amateur  photographs  is  that  obtained  when  the 
equivalent  focus  of  the  lens  is  equal  to  the  diagonal  of 
the  plate.  Most  makers  list  anastigmats  of  shorter 
focus  than  this  for  each  plate,  for  instance  6  inches 
instead  of  6|  inches  for  4  X  5,  7  inches  instead  of  83 


AND  USE  A  LENS.  49 

inches  for  5  X  7,  and  so  on.  In  such  a  case  it  is  often 
better  to  select  the  next  size  larger.  A  greater  focal 
length  is  of  advantage  from  a  pictorial  standpoint, 
hence  the  popularity  of  symmetrical  and  convertible 
anastigmats,  the  halves  of  which  can  be  used  as 
single  view  lenses,  if  the  camera  has  a  long  bellows. 
If,  however,  one  intends  to  do  only  the  usual  types  of 
vacation  pictures,  snapshots  of  the  children  at  play, 
and  an  occasional  out-door  figure  study  or  indoor 
portrait,  a  lens  of  focal  length  equal  to  the  diagonal 
will  be  the  best  compromise,  and  the  speed  should 
depend  on  the  camera  and  the  shutter.  With  a  re- 
flecting camera  of  any  size,  the  use  of  a  lens  working 
at/4-5  is  decidedly  to  be  recommended  because  this 
gives  the  utmost  possibility  for  speed  work.  When  a 
faster  lens  is  stopped  down  to  the  same  aperture  as  a 
slower  one  of  the  same  focal  length,  their  performance 
is  much  the  same,  though  the  slower  one  will  prob- 
ably cover  a  larger  field.  The  greatest  disadvantage 
of  large  aperture  (that  is  speed)  is  the  diminution 
of  the  depth  of  sharp  field.  The  nearer  the  object 
focused,  the  shallower  the  depth  of  field,  hence  the 
practical  necessity  of  using  a  reflecting  camera  for  an 
/:4.5  lens  of  moderate  focal  length.  If,  however, 
much  portraiture  in  the  home  is  to  be  done  with  any 
type  of  camera,  the  advantages  of  the  /:4-5  lens  are 
so  great  that  it  should  unhesitatingly  be  chosen  in 
focal  lengths  up  to  about  9  inches,  at  any  rate.  As 
a  portrait  lens  alone,  this  speed  is  not  too  great  in 
lenses  of  even  longer  focus  —  in  fact,  for  this  special 
use,  there  is  a  great  advantage  in  using  as  large  a  lens 
as  can  be  worked  in  the  available  space.  For  in- 
stance, a  1 6-inch  lens  at  16  feet  will  give  better  per- 


50  HOW  TO  CHOOSE 

spective  than  an  8-inch  lens  at  8  feet,  though  the  two 
images  will  be  of  the  same  size. 

Types.  —  The  choice  between  the  different  types 
of  lenses  is  very  largely  a  matter  of  taste;  for  the 
rest,  as  already  hinted,  the  use  of  a  symmetrical  or  a 
convertible  lens  on  a  single-extension  camera  is  no 
better  than  a  tying-up  of  valuable  properties  which 
cannot  be  used.  The  user  may,  however,  some  day 
get  another  camera  on  which  he  can  use  the  single 
combinations,  so  there  is  something  to  be  said  in 
favor  of  planning  for  the  future.  Otherwise,  there 
are  lenses  of  thoroughly  satisfactory  correction  in 
both  classes. 

Using  an  Anastigmat.  First  Perplexities.  —  When 
one  has  been  using  a  rectilinear,  particularly  when  it 
has  been  employed  chiefly  for  tripod  work  with 
medium  stops,  the  first  impression  of  an  anastigmat 
at  full  opening  is  that  it  has  very  little  depth  and 
needs  stopping  down.  The  image,  where  it  is  in  per- 
fect focus,  is  so  much  more  clean-cut  and  brilliant  than 
anything  ever  seen  before  that  the  out-of-focus  parts 
look  distressingly  soft  and  fuzzy.  The  second  thing 
noticed  is  that  a  very  minute  movement  of  the  lens 
is  sufficient  to  throw  the  image  out.  If  the  lens  has 
been  put  on  an  old  camera,  with  a  front  which  is 
shaky  from  wear,  one  side  of  the  picture  may  be  in 
perfect  focus  and  the  other  blurred.  A  perfect  lens 
must  have  a  perfect  camera.  The  front  must  be 
rigid  and  hold  the  lens  with  its  axis  perpendicular 
to  the  film,  or  the  definition  will  be  poorer  than  that 
obtained  with  the  rapid  rectilinear.  When  using  the 
focusing  scale,  extreme  care  must  be  taken  to  set 
the  pointer  accurately  to  the  required  distance.  An 


AND  USE  A  LENS.  51 

error  of  a  few  hundredths  of  an  inch  may  be  suffi- 
cient to  make  the  difference  between  crisp,  spark- 
ling detail  and  a  general  softness  which  ruins  the 
effect.  Finally,  one  discovers  that  when  a  flat  sur- 
face is  focused,  the  detail  is  as  good  in  the  corners 
as  at  the  centre  and  one  does  not  have  to  split  the 
focus,  as  the  field  is  perfectly  flat.  This  brings  us  to 
the  conclusion  that  the  lens  will  need  stopping  down 
only  when  it  is  necessary  to  increase  the  depth  of  the 
sharp  field,  not  to  secure  covering  power. 

Securing  the  Anastigmat  Advantages.  —  Other 
things  being  equal,  an  anastigmat  in  a  poor,  cheap 
shutter  is  not  able  to  do  its  best  work.  Here  we  have 
a  usable  aperture  of  say,  /:6.3,  and  a  nominal  Tta 
second  which  may  be  really  ^.  In  bright  summer 
sunlight  it  is  necessary  to  stop  down  in  order  to  avoid 
over-exposure,  or  conversely,  we  cannot  take  a  sharp 
picture  of  athletic  sports,  because  we  lack  the  needful 
iffa  second  actual  exposure  which  a  better  shutter 
gives.  Again,  the  low-priced  shutter  has  no  speed 
between  -fa  and  £  second.  One  is  too  fast  for  a  suc- 
cessful snapshot  at  full  aperture  in  poor  light,  the 
other  too  slow  for  the  camera  to  be  held  in  the  hand 
without  causing  blur.  The  costlier  shutter,  how- 
ever, has  an  accurate  TV  second  which  is  just  what 
is  needed.  In  short,  when  you  get  your  anastigmat, 
have  it  mounted  in  the  best  shutter  you  can  afford, 
if  you  are  not  purchasing  a  camera  fitted  with  a 
focal-plane  shutter.  It  is  well  to  remark  that  a  high- 
grade  shutter  with  really  accurate  speeds  may  prove 
a  difficult  tool  to  become  acquainted  with  after  using 
a  cheap  and  inaccurate  one,  leading  to  serious  under- 
exposure if  used  without  realizing  the  fact  that  the 


52  HOW  TO  CHOOSE 

speeds  may  be  twice  as  great  as  those  of  the  same 
markings  on  the  cheap  shutter.  Occasionally  it  is 
an  advantage  to  have  the  lens  in  a  shutter  instead  of 
a  barrel  even  on  a  reflecting  camera,  as  the  focal- 
plane  shutter  of  the  latter  is  not  so  well  suited  to 
give  slow  snapshots  and  time  exposures  as  the  be- 
tween-lens  shutter.  Extra  speed  in  the  lens  itself  is 
not  of  much  use  if  it  cannot  be  made  available  by  a 
wide  range  of  shutter  speeds. 

Focusing.  —  Anastigmats  being  perfectly  achro- 
matic for  two  and  usually  for  three  colors,  the  image 
seen  on  the  ground  glass  is  exactly  what  one  gets 
on  the  plate,  if  the  latter  is  in  register  with  the 
former.  Sometimes  a  fine  lens  gives  poor  results, 
and  the  trouble  is  found  to  be  due  to  the  fact  that  the 
holders  are  not  bringing  the  plate  within  a  sensible 
distance  of  the  same  plane  as  the  ground  side  of 
the  glass.  In  such  cases,  the  holders  must  be  tested 
by  means  of  a  straight-edge  and  a  wedge,  and  the 
position  of  the  ground  glass  altered  to  correspond. 
Users  of  reflecting  cameras  may  require  special 
spectacle  lenses  fitted  inside  the  hood  in  order  to 
focus  accurately.  Incorrect  register  of  the  ground 
glass  is  not  unknown,  even  in  costly  reflecting  instru- 
ments. But  when  the  register  has  been  proved  cor- 
rect, one  often  has  difficulty  in  learning  to  focus. 
The  tendency  is  to  move  the  focusing  screw  too 
rapidly.  The  difference  between  a  microscopically 
sharp  focus  and  a  blur  may  be  one  of  a  touch  of  the 
pinion-head.  Care  must  be  taken  not  to  focus  too 
far  back,  thus  sharpening  the  distance  at  the  ex- 
pense of  the  foreground  and  making  the  latter  un- 
pleasantly blurry  by  contrast  with  the  extreme, 


AND   USE  A  LENS.  53 

needle-point  definition  of  the  background.  Stopping 
down,  however,  will  be  needed  less  often  than  one 
would  at  first  expect,  as,  owing  to  the  perfect  flatness 
of  field,  one  can  focus  at  any  point  of  the  picture, 
and  then  carefully  divide  the  sharpness  between  far 
and  near  objects,  favoring  the  foreground  as  much  as 
possible,  before  putting  in  a  smaller  stop.  Generally, 
if  the  plane  of  sharpest  focus  is  one-third  of  the  dis- 
tance from  camera  to  farthest  object,  a  medium  stop 
will  give  sufficient  depth;  but  if  the  plane  of  sharp 
focus  were  placed  at  two-thirds  of  this  distance,  the 
smallest  stop  would  often  hardly  suffice. 

Scale  Focusing.  —  When  the  camera  is  sold  fitted 
with  an  anastigmat,  the  correctness  of  its  setting 
should  be  tried  by  making  an  exposure  at  the  exact 
measured  distance  for  each  mark  on  the  focusing 
scale.  In  making  these  tests  be  sure  that  the  front 
is  solidly  locked  or  drawn  out  to  the  stop,  or  the 
pointer  set  exactly  on  the  mark.  Should  the  scale  prove 
to  be  incorrectly  placed,  adjust  it  by  careful  trial. 

Hyperfocal  Distances.  —  The  greatest  possible 
depth  of  sharp  field  is  obtained  when  a  lens  is  set  to  a 
definite  distance  for  each  stop.  The  distance  is 
found  by  the  formula  given  on  page  17.  Suppose  a 
6-inch  lens  works  at  /:4-5-  Its  hyperfocal  distance 
for  this  stop  is  66  feet.  In  the  same  manner,  the 
distances  for  the  other  stops  may  be  worked  out  and 
tabulated,  a  trial  made  at  these  measured  distances, 
and  a  supplementary  scale  made  for  the  camera. 
Such  a  scale  is  far  more  useful  than  one  marked  with 
conventional  distances.  To  use  it,  set  the  pointer  to 
the  distance  corresponding  to  the  stop  in  use,  and 
the  depth  of  sharp  field  will  extend  from  half  the 


54  HOW  TO  CHOOSE 

hyperfocal  distance  to  infinity.  Thus,  with  the  lens 
already  mentioned,  the  66-foot  setting  will  give  a 
sharp  image  of  everything  more  than  33  feet  from 
the  lens.  The  same  lens  at  /:8  (which  we  consider 
the  normal  stop  for  hand-camera  snapshots)  may  be 
set  at  38  feet,  and  will  give  good  definition  from  19 
feet  to  the  horizon.  It  is  by  applying  this  rule  to 
the  /:i6  meniscus  of  short  focus  that  the  manu- 
facturers produce  the  so-called  fixed-focus  lens. 

Other  Points  About  Anastigmats.  —  We  might 
write  many  pages  on  the  use  of  anastigmat  lenses 
without  covering  all  the  points  which  might  arise. 
The  owner  of  one,  however,  will  learn  more  from 
using  it  than  from  reading  directions.  The  chief  ad- 
vantages of  the  anastigmat  are  that  its  corrections 
have  been  carried  out  so  that  it  condenses  to  a  very 
small  circle  of  light  in  the  image,  each  bundle  of  rays 
originating  in  a  point  of  the  object,  whether  the  ray 
passes  centrally  or  obliquely  through  the  lens.  This 
circle  of  confusion,  as  it  is  called,  should  have  a 
diameter  of  about  T^TF  of  an  inch  in  a  rapid  rectilinear, 
and  only  from  ^^  to  -5^  in  an  anastigmat.  This 
means  in  practice  that  an  anastigmat  will  give  a 
critically  sharp  image  over  the  entire  plate  at  a  large 
aperture. 

Care  of  Lenses.  —  A  fine  instrument  requires  care- 
ful handling.  When  not  in  use  the  lens  should  be 
kept  in  a  stout  box,  with  a  cap  over  each  end,  in  a 
cool  place.  Dust  may  be  removed  from  the  com- 
binations with  a  clean  camel's  hair  brush,  holding  the 
lens  up  and  brushing  it  very  gently  from  underneath 
so  that  the  dust  will  fall  off.  If  a  greasy  film  collects 
on  the  glasses,  they  may  be  unscrewed  and  very 


AND  USE  A  LENS.  55 

gently  wiped  with  the  Japanese  tissue  paper  sold  by 
optical  and  physicians'  supply  houses,  under  the 
name  of  lens  paper.  Rubbing  must  be  avoided. 
Glass  surfaces  are  relatively  very  soft.  If  one  rubs 
the  surface  with  a  dusty  cloth,  the  polish  may  be  de- 
stroyed or  the  curvatures  altered,  ruining  the  lens. 
No  other  substance  than  lens  paper  should  ever  be 
allowed  to  touch  the  surfaces  of  the  glasses.  The 
greatest  care  must  be  exercised  in  taking  apart  and 
putting  together  air-space  anastigmats.  With  some 
of  these  an  error  of  adjustment  of  ^iv  of  an  inch 
will  cause  considerable  deterioration  of  the  quality 
of  the  image.  A  lens  may  require  recementing  after 
years  of  use,  or  repolishing,  if  the  softer  varieties  of 
Jena  glass  assume  a  rainbow-hued  appearance.  In  any 
such  case,  send  it  to  its  maker  for  the  necessary  repairs. 

Special  Fields  of  Photography.  —  So  far,  we  have 
discussed  only  the  ordinary  kinds  of  photography, 
but  many  prospective  purchasers  of  a  lens  may  wish 
to  use  it  for  such  special  fields  as  portraiture,  copying, 
or  enlarging.  The  "  how  to  use  "  part  is  so  different 
that  it  requires  a  little  consideration. 

Portraiture.  —  Although  good  portraits  can  be 
made  with  a  simple  outfit,  there  are  certain  diffi- 
culties which  can  be  overcome  only  by  knowing  all 
the  tricks  and  having  a  camera  provided  with  all 
possible  movements,  inasmuch  as  the  use  of  a  lens 
very  near  to  a  subject  which  has  considerable  por- 
tions projecting  forward  and  back  of  the  main  plane 
focused  on  may  lead  to  poor  results.  Short  exposures 
are  desirable,  which  means  using  a  large  aperture 
and  thereby  losing  depth  of  field.  The  deficiency 
must  therefore  be  made  up  not  only  by  tilting  the 


56  HOW  TO  CHOOSE 

camera  to  point  down  at  the  sitter,  but  also  by  uti- 
lizing the  vertical  and  horizontal  swings  to  equalize 
definition  between  nearer  and  farther  parts.  For 
instance,  if  a  sharp  focus  is  secured  on  the  sitter's 
eye,  with  the  ground  glass  vertical,  it  will  be  found 
impossible  to  secure  sharpness  in  a  hand  resting  on 
the  lap,  unless  the  lens  is  stopped  down  or  the  vertical 
swing  is  used  to  bring  the  ground  glass  away  from 
the  vertical  plane  to  the  place  where  the  sharp  image 
of  the  hand  really  lies.  Similarly,  both  shoulders  can 
be  focused,  in  a  three-quarter  pose,  only  by  using  the 
horizontal  swing.  For  these  reasons  a  good  view 
camera  will  be  found  the  best  purchase  for  an  ama- 
teur wishing  to  do  much  portraiture.  Since  vertical 
lines  seldom  appear  in  a  portrait,  the  ground  glass 
may  be  placed  in  any  position  where  it  will  receive  a 
sharp  image.  The  nearer  any  object  is  to  the  lens, 
the  farther  its  image  lies  from  the  lens. 

Copying  and  Enlarging.  —  The  last  statement  gov- 
erns the  use  of  the  lens  for  copying  and  enlarging. 
The  ordinary  camera  with  a  single  extension  of  bel- 
lows can  be  used  to  focus  objects  only  up  to  about 
6  feet.  If  a  larger  image  is  desired,  it  must  be  obtained 
through  the  use  of  a  supplementary  lens  ("  portrait 
attachment "  or  "  copying  lens "),  or  by  using  a 
camera  with  double  or  triple  extension  of  the  bellows. 
For  instance,  to  copy  a  photograph  "  same  size  "  with 
a  7-inch  lens  (about  the  focus  of  that  supplied  on  a 
33  film  camera)  the  lens  must  be  14  inches  from  the 
ground  glass  and  14  inches  away  from  the  "  copy.1' 
To  secure  an  enlargement  of  4  diameters  with  the 
same  lens,  that  is,  to  enlarge  from  3i  X  s|  to  13  X  22, 
the  lens  would  have  to  be  8|  inches  from  the  negative 


AND   USE  A  LENS.  57 

and  35  inches  from  the  bromide  paper  on  the  easel. 
As  the  bellows  is  too  short  for  such  use,  an  enlarging 
apparatus  would  have  to  include  a  box  behind  the 
kodak  of  such  length  as  to  secure  the  necessary  8| 
inches'  extension  —  or  more,  if  smaller-scale  enlarge- 
ments were  desired.  Suffice  it  to  say,  it  is  useless  to 
purchase  the  most  universal  anastigmat  unless  it  is 
fitted  to  a  camera  having  adequate  draw  of  bellows. 
As  a  matter  of  fact,  unless  one  confines  photographic 
activity  to  outdoor  hand-camera  work,  it  will  be 
found  that  two  or  more  cameras  of  different  sizes  and 
capabilities  are  needed  to  cover  every  department  of 
photography  successfully. 

Wide-angle  Lenses.  —  A  lens  capable  of  producing 
a  negative  embracing  an  angle  of  view  greater  than 
70°  is  called  a  wide-angle  lens.  Up  to  this  angle, 
although  it  is  much  greater  than  that  seen  by  the 
eye  at  once,  the  perspective  is  not  too  extreme,  but 
above  this  angle,  objects  near  the  edge  of  the  picture 
are  much  distorted,  and  such  lenses  should  be  used 
only  where  cramped  situations  render  their  use  oblig- 
atory. A  6-inch  rapid  rectilinear,  for  instance,  will 
work  fairly  well  on  a  4  X  5  plate,  including  56°  along 
the  diagonal  of  the  plate.  If  the  same  lens  were 
used  on  a  5  X  7  plate  the  angle  included  along  the 
diagonal  would  be  71°,  and  the  lens  in  this  instance 
would  be  a  wide-angle  lens  for  that  plate.  Ordinary 
rectilinears,  however,  are  not  well  adapted  to  such 
use;  a  special  form,  made  of  heavier  glasses  and  with 
deeper  curvatures,  working  at/:i6  or  slower,  can  be 
bought.  Cells  to  fit  ordinary  shutters  and  lenses  in 
barrel-mounts  are  still  obtainable  and  are  useful  ad- 
ditions to  an  outfit,  particularly  for  copying  or  for 


58  HOW  TO  CHOOSE 

taking  interiors  or  other  close  subjects  when  the  regu- 
lar lens  cannot  be  placed  far  enough  from  the  subject 
to  include  enough  of  it.  For  instance,  suppose  one 
has  a  5  X  7  camera  with  single  extension  of  bellows 
and  fitted  with  an  8j-inch  rapid  rectilinear  lens.  A 
set  of  wide  angle  cells,  listed  at  $6,  will  furnish,  for 
the  same  shutter,  a  lens  having  an  equivalent  focus 
of  about  5J  inches.  Such  a  lens  would  include  78° 
on  the  diagonal,  allowing  its  use  for  small  interiors. 
It  could  also  be  used  to  make  a  copy  half  the  size  of 
the  original  with  the  short  bellows  furnished  on  the 
camera,  whereas  the  regular  rapid  rectilinear  would  not 
make  one  larger  than  £  size  with  the  same  extension. 

Limitations  of  the  Old  Rapid  Rectilinears.  —  The 
special  wide-angle  types  of  rectilinears,  of  course,  have 
all  the  defects  of  their  class,  with  the  added  disad- 
vantage of  a  small  maximum  opening  for  focusing. 
Still,  if  they  are  carefully  used,  they  are  capable  of 
doing  remarkably  fine  work  with  very  small  stops. 
It  is  necessary  to  split  the  curvature  of  field  when 
focusing,  and  in  interior  work,  to  make  sure  that  the 
proper  depth  is  secured  by  focusing  wide  open  on 
some  subject  only  a  third  of  the  whole  depth  of  the 
subject  from  the  lens.  For  instance,  if  the  interior 
measures  30  feet  from  lens  to  most  distant  wall,  a 
point  10  feet  from  the  lens  should  be  chosen  for  the 
sharpest  focus  at  full  aperture  and  the  lens  then  be 
stopped  down  until  everything  is  sharp  enough. 

Modern  Wide-Angles.  —  Some  of  the  earlier  an- 
astigmats  were  wide-angles,  notably  the  Zeiss  Series 
V.  Several  other  special  forms  of  anastigmat  wide- 
angle  lenses  have  been  marketed,  but  the  symmetrical 
cemented  double  anastigmats,  as  well  as  some  of  the 


AND  USE  A  LENS.  59 

unsymmetrical  objectives,  answer  the  purpose  so  per- 
fectly that  they  are  generally  chosen.  For  instance, 
a  6-inch  lens  which  is  intended  for  a  4  X  5  plate  will 
cover  5X7  at/:i6  and  6|  X  8J  at/:22  or/32.  In 
planning  for  two  cameras  of  different  sizes,  therefore, 
it  is  not  inadvisable  to  select  for  the  smaller  instru- 
ment a  lens  which  will  answer  for  use  as  a  wide-angle 
on  the  larger  camera. 

Teleobjectives.  —  When  it  is  necessary  to  obtain 
a  large  direct  picture  of  a  distant  object,  in  order  to 
avoid  lenses  of  extreme  focal  length,  with  corre- 
spondingly long  bellows,  lens  systems  are  used  which 
contain  a  negative  or  dispersing  lens  in  combination 
with  a  collecting  lens.  Telephoto  attachments  may 
be  had  for  either  rapid  rectilinear  or  anastigmat 
lenses,  and  give  various  magnifications  by  varying 
the  separations  of  the  lenses.  They  greatly  increase 
the  focal  length  of  the  lens,  and  therefore  reduce  the 
effective  aperture  or  /  value,  so  that  exposures  must 
be  very  long. 

Limitations  of  Telephotography.  —  It  is  difficult  to 
obtain  good  pictures  with  the  long  exposures  needed 
at  high  magnifications.  The  camera  itself  must  be 
solidly  built  to  obviate  vibration,  and  the  air  must  be 
as  still  as  possible.  There  are,  on  the  whole,  very 
few  days  in  the  year  which  will  allow  successful  tele- 
photographic  work.  The  greater  the  enlargement, 
the  more  noticeable  is  the  effect  of  air  currents  in 
blurring  the  detail.  The  lower  amplifications,  4^  to 
6  times,  are  more  likely  to  prove  usable. 

Fixed-magnification  Teleobjectives. — Practical  con- 
siderations, such  as  have  been  mentioned,  led  to  the 
abandonment  of  the  variable  magnification  and  the 


60  HOW  TO  CHOOSE 

construction  of  teleobjectives  suitable  for  reflecting 
and  hand  cameras  and  having  a  fixed  magnification 
of  about  2  or  2\  diameters.  In  other  words,  these 
lenses  give  a  large  image  with  short  bellows  extension. 
One  of  them,  the  Ross  Telecentric,  gives  almost  as 
fine  definition  as  an  anastigmat  and  has  a  working 
aperture  of  /:54.  The  chief  advantage,  as  com- 
pared with  a  long-focus  regular  lens  having  the  same 
equivalent  focus,  is  the  rigidity  of  the  short  bellows. 
The  17-inch  Telecentric  /:54,  for  instance,  has  a 
back  focus,  or  distance  from  rear  lens-cell  to  ground 
glass,  of  only  9TV  inches.  It  could  thus  be  ad- 
vantageously fitted  to  a  4X5  reflecting  camera 
which  could  hardly  accommodate  a  1 7-inch  Tessar,  for 
instance,  even  if  the  photographer  could  afford  to 
pay  for  the  latter.  Other  manufacturers  brought 
out  various  forms  of  the  telephoto  objective,  among 
which  may  be  mentioned  the  Cooke  Telar,  the  Dall- 
meyer  Adon,  and  the  Zeiss  Magnar. 

Trying  a  Lens.  —  Most  workers  in  buying  lenses 
are  interested  in  those  of  the  highest  possible  cor- 
rections and  desire  to  know  just  how  they  can  make 
a  wise  selection  from  the  many  varieties  offered. 
Since  the  prices  of  anastigmat  lenses  have  come  down, 
the  difficulty  of  making  a  choice  has  increased.  One 
worker  is  willing  to  pay  any  sum  if  he  can  be  shown 
that  the  lens  is  really  better  than  some  other  of  equal 
or  lower  cost.  Another  worker  is  strictly  limited  by 
his  pocketbook,  but  doubts  whether  the  price  he  can 
afford  will  actually  buy  a  genuine  anastigmat  of  good 
corrections  and  capable  of  doing  well  all  the  sorts  of 
photographic  work  in  which  he  is  interested.  The 
following  paragraphs  give,  for  the  benefit  of  both 


AND  USE  A  LENS.  61 

classes,  some  methods  of  testing  lenses,  based  on  an 
article  published  in  The  British  Journal  of  Photog- 
raphy. By  trying  the  lenses,  usually  sent  with  the 
privilege  of  a  ten  days'  trial  by  the  makers,  the  reader 
can  at  slight  trouble  determine  just  how  good  they  are. 

Testing  a  Lens.  —  There  are  two  distinct  things 
which  have  to  be  ascertained  about  a  lens  before 
taking  it  into  use  —  its  capabilities  and  its  qualities. 
Under  the  heading  of  capabilities  are  three  principal 
points:  The  intensity,  or,  as  some  call  it,  the  speed  at 
full  aperture ;  the  relation  of  the  circle  of  illumination 
to  focal  length  or  angle  of  view;  and  whether  the 
image  is  sufficiently  rectilinear  for  the  work  the  lens  is 
to  be  used  for.  Want  of  rectilinearity  is  not  a  defect 
for  some  work:  it  may  even  be  an  advantage,  so  we 
place  this  point  under  capabilities  instead  of  qualities. 

Focal  Length.  —  To  arrive  at  the  intensity,  or,  as 
it  is  often  called,  angular  aperture,  we  must  first 
ascertain  with  some  degree  of  accuracy  the  actual 
or  equivalent  focal  length  of  the  lens.  There  are 
many  ways  of  doing  this,  perhaps  the  simplest  being 
to  focus  a  distant^object,  to  mark  the  camera  ex- 
tension on  the  baseboard,  and  then  to  focus  any 
object,  such  as  a  printed  card,  so  that  the  image  on 
the  ground  glass  is  exactly  the  size  of  the  original. 
The  difference  between  the  original  extension  as 
marked  and  the  extension  required  for  a  full-sized 
image  is  the  equivalent  focal  length  of  the  lens. 

Focal  Aperture.  —  The  intensity  is  got  by  dividing 
the  diameter  of  the  largest  aperture  of  the  diaphragm 
into  the  focal  length.  This,  however,  although  not 
difficult  of  accomplishment,  is  not  quite  as  easy  as  it 
sounds.  As  in  obtaining  the  focal  length,  it  is  not 


62  HOW  TO  CHOOSE 

enough  to  measure  the  distance  between  the  lens  and 
plate  with  a  rule,  so,  in  measuring  the  aperture,  it 
is  not  correct  to  measure  the  diaphragm  opening  with 
a  pair  of  dividers.  In  many  lenses  there  is  a  con- 
siderable convergence  of  the  rays  before  they  reach 
the  diaphragm,  so  that  it  is  quite  possible  for  a  lens 
having  an  actual  aperture  of  /:6.3  to  seem  to  have 
one  no  larger  than  f:j.  To  ascertain  the  amount  of 
this  convergence,  we  must  focus  a  distant  object 
upon  the  screen,  and  after  replacing  the  ground  glass 
by  a  card  which  has  had  a  fairly  large  pinhole  per- 
forated in  the  centre,  take  the  camera  into  a  darkened 
room  and  hold  a  candle  flame  close  to  the  pinhole. 
If  a  piece  of  ground  glass  be  placed  in  contact  with 
the  front  cell  or  hood  of  the  lens,  we  shall  see  an 
illuminated  circle,  .and  this  circle  is  the  true  working 
aperture  of  whatever  diaphragm  aperture  happens 
to  be  in  position.  In  a  general  way,  we  should  start 
with  the  largest  aperture;  we  may  then  proceed  to 
check  all  the  others.  As  it  is  somewhat  difficult  to 
measure  the  disc  of  light  while  holding  the  ground 
glass  in  position,  it  is  a  good  plan  to  rule  a  pencil  line 
upon  it,  and  mark  off  upon  that  the  various  aper- 
tures. A  better  way  is  to  place  a  piece  of  gaslight 
paper  in  the  same  position,  and  after  developing, 
measure  the  diameter  of  the  blackened  circle.  This 
gives  a  permanent  record. 

Circles  of  Illumination.  —  It  is  not  quite  so  easy 
to  determine  the  size  of  the  circle  of  illumination  with 
a  lens  unless  one  has  access  to  a  very  large  camera. 
With  a  4  X  5  lens,  a  10  X  12  camera  will  answer,  but 
for  lenses  of  over  8  or  9  inches  focal  length,  some 
special  device  will  be  necessary.  An  optician's  test- 


AND   USE  A  LENS.  63 

ing-bench  is,  of  course,  the  most  convenient  thing 
for  this  purpose,  but  is  hardly  likely  to  be  available. 
The  easiest  way  is  to  fasten  the  lens  to  a  fairly  large 
front  board  and  to  fix  this  in  a  window  the  rest  of 
which  can  be  covered  with  a  dark  curtain.  A  child's 
hoop  of  about  30  inches  in  diameter,  covered  with 
tracing  paper,  will  serve  as  a  focusing  screen,  and  if 
a  scale  of  inches,  starting  with  zero,  in  the  centre,  is 
marked  on  the  paper,  it  will  at  once  show  the  limits 
of  the  field.  Here,  again,  a  sheet  of  bromide  or  gas- 
light paper  on  a  movable  easel  is  preferable.  The 
lens  should  be  focused  on  infinity  for  this  test.  To 
ascertain  the  covering  power  for  any  size  of  plate,  it 
is  a  good  plan  to  draw  a  circle  the  size  of  the  field  of 
the  lens,  and  to  lay  an  actual  plate  upon  it.  It  is 
then  easy  to  see  how  much  rise  and  fall  is  possible 
without  any  further  measurement  or  calculation. 

The  test  for  rectilinearity  should  be  made  on  the 
class  of  subject  it  is  desired  to  take,  as  it  is  a  fact 
that  lenses  which  are  rectilinear  at  some  distances 
may  not  be  so  at  others.  We  once  possessed  a  lens 
wrhich  was  absolutely  correct  on  a  test  chart  at  a  few 
feet,  but  which  gave  a  perceptible  curvature  of  the 
field  in  a  church  interior.  The  test  should  also  be 
made  on  the  size  of  the  plate  to  be  used,  as  some 
single  lenses  give  practically  no  distortion  over  a 
moderate  angle.  The  test  should  take  the  form  of 
an  actual  exposure,  as  it  is  much  easier  to  examine  a 
negative  than  an  image  upon  the  ground  glass. 

Qualities  of  the  Lens.  —  Before  attempting  to 
make  any  tests  as  to  the  quality  of  a  lens,  the  camera 
upon  which  it  is  to  be  fixed  should  be  carefully  ex- 
amined. The  larger  the  aperture  and  the  more 


64  HOW  TO  CHOOSE 

highly  corrected  the  lens  is  supposed  to  be,  the  more 
necessity  exists  for  this.  Most  present-day  cameras 
are  very  lightly  built,  and  a  slight  bending  of  the 
struts  or  straining  of  the  hinges  will  cause  a  lack  of 
parallelism  between  the  front  and  back  which  will 
seriously  impair  the  accuracy  of  the  result. 

The  principal  defects  which  have  to  be  looked  for 
are  spherical  and  chromatic  aberrations,  astigmatism, 
curvature  of  field,  and  flare.  Besides  these,  the 
quality  of  the  glass  must  be  examined  for  specks. 
Faults  in  mounting,  such  as  bad  centering  and  strain 
through  too  tightly  fitting  the  glasses  into  the  cells, 
may  also  be  present. 

Spherical  Aberration.  —  Spherical  aberration  means 
incapacity  to  give  sharp  definition  at  a  large  aper- 
ture. It  arises  from  the  fact  that  unless  properly 
corrected,  the  rays  passing  through  the  margin  of  a 
lens  have  a  slightly  different  focal  length  from  those 
passing  through  the  centre.  This  causes  a  softness 
of  outline  which  is  most  noticeable  around  the  highest 
lights  of  the  subject,  such  as  the  white  collar  in  a 
portrait  or  a  patch  of  white  wall  in  a  view.  It  may 
be  recognized  by  focusing  a  printed  page  from  a 
good  magazine,  on  which  the  ink  is  black  and  the 
paper  smooth.  If  it  is  impossible  to  secure  a  sharp 
image  in  the  center  of  the  field  without  stopping 
down  the  lens,  spherical  aberration  is  present.  Ex- 
cept in  very  bad  cases,  a  sharp  image  can  be  secured 
when  the  aperture  is  reduced  to  /:i6.  Stopping  down 
an  R.  R.  lens  may  not  remove  astigmatism,  however. 
A  more  delicate  test  is  to  use  the  light  of  a  small  lamp 
reflected  upon  a  mercury  bulb  (an  ordinary  thermom- 
eter will  do  as  the  test  object)  in  a  darkened  room, 


AND   USE  A  LENS.  65 

as  the  blurring  is  shown  more  clearly.  Another  way 
which  also  demonstrates  the  cause  as  well  as  proves 
the  existence  of  this  aberration  is  to  make  two  stops 
of  black  paper.  One  is  a  disc  about  two-thirds  the 
diameter  of  the  full  aperture  of  the  lens,  the  other  a 
disc  which  will  fit  inside  the  lens  hood,  and  having  a 
central  perforation  of  a  diameter  a  little  larger  than 
the  solid  disc.  The  perforated  disc  is  placed  in  the 
hood,  and  any  object  sharply  focused,  the  exact 
position  of  the  screen  being  marked.  This  disc  is 
then  removed,  and  the  other  disc  fixed  on  the  centre 
of  the  front  lens  with  a  touch  of  gum  or  rubber  solu- 
tion. If  spherical  aberration  is  present,  the  image 
will  no  longer  be  sharp,  but  it  can  be  made  so  by  re- 
focusing.  If  poor  definition  is  due  to  other  causes, 
such  as  bad  centering  or  badly  annealed  glass,  there 
will  be  little  difference  in  the  definition  when  the 
discs  are  changed.  It  should  be  remembered  that 
for  some  classes  of  work  spherical  aberration  is  ad- 
vantageous, and  special  means  for  introducing  and 
regulating  it  are  provided  in  the  Patent  Portrait 
lenses  of  Dallmeyer,  and  more  recently  in  the  Cooke 
Anastigmat  Portrait  objectives.  In  either  of  these, 
by  simply  turning  one  of  the  lens  cells,  a  range  from 
absolute  sharpness  to  decided  fuzziness  may  be  ob- 
tained. The  Wollensak  Velostigmat  Series  II  has  a 
similar  adjustment  in  the  larger  sizes. 

Astigmatism.  —  Astigmatism  has  been  already  de- 
fined. It  differs  from  spherical  aberration  inasmuch 
as  it  cannot  be  wholly  removed  by  any  amount  of 
stopping  down.  It  is  usually  present  to  a  large  ex- 
tent in  portrait  lenses  of  the  Petzval  type,  and  to  a 
lesser  one  in  a  rapid  rectilinear.  The  handiest  test 


66  HOW  TO  CHOOSE 

for  it  is  a  cross  made  of  two  strips  of  black  paper 
about  three  inches  long  and  a  quarter  of  an  inch  wide, 
pasted  upon  a  white  card.  If  this  be  focused  upon 
the  center  of  the  ground  glass,  the  vertical  and  hori- 
zontal lines  will  appear  equally  sharp,  but  upon  ro- 
tating the  camera  so  as  to  bring  the  image  near  the 
edge  of  the  plate,  it  may  appear  unsharp.  This  may 
be  due  to  curvature  of  the  field,  in  which  case,  a 
sharp  image  can  be  obtained  by  refocusing.  If  astig- 
matism be  present,  it  will  be  found  that  only  one  of 
the  lines  can  be  focused  at  a  time;  when  the  verti- 
cal one  is  sharp,  the  horizontal  one  is  fuzzy,  and  vice 
versa.  If  the  mercury  bulb  or  "  artificial  star  "  be 
used  for  this  test,  the  bright  dot  focused  in  the  centre 
of  the  field  is  drawn  out,  on  rotating  the  camera,  into 
a  short  line,  which  takes  a  horizontal  or  a  vertical 
position  as  the  camera  back  is  racked  out  or  in. 

Chromatic  Aberration.  —  Chromatic  aberration  is 
a  serious  defect  for  ordinary  work.  A  lens  afflicted 
with  it  will  give  a  sharp  image  upon  the  ground  glass; 
but,  upon  taking  a  negative,  the  image  of  the  object 
appears  unsharp,  while  other  objects  at  a  greater  or 
less  distance  appear  sharper  than  they  did  upon  the 
screen.  This  is  due  to  the  fact  that  the  yellow  rays, 
which  are  the  most  luminous,  are  those  which  are 
mainly  used  in  focusing  and  produce  what  is  known 
as  the  visual  image,  while  the  blue  and  violet  rays, 
which  are  more  chemically  active,  come  to  focus  in 
another  place.  The  simplest  test  for  a  lack  of  achro- 
matism is  to  fix  up  a  long  strip  of  sharply-defined 
printed  matter  at  an  angle  with  the  axis  of  the  lens, 
the  centre  of  the  strip  being  opposite  the  lens;  in  a 
strip  of  two  feet  of  length  the  inclination  should  make 


AND   USE  A  LENS.  67 

the  nearest  end  of  the  strip  about  eight  inches  closer 
to  the  camera  than  the  further  one.  It  is  convenient 
to  place  a  couple  of  pins  to  mark  the  centre  of  the 
print.  A  column  of  newspaper  answers  well  for  this 
purpose.  Then  focus  the  line  lying  between  the  pins, 
and  make  a  negative.  If  the  line  focused  is  the 
sharpest  in  the  negative,  the  lens  is  free  from  chro- 
matic error;  if  it  be  undercorrected,  a  more  distant 
line  will  be  in  the  best  focus;  if  over-corrected,  a 
nearer  one.  A  properly  achromatized  lens  brings 
both  blue  and  yellow  rays  to  the  same  focal  plane, 
while  an  apochromatic  one  makes  practically  all  the 
rays  of  the  spectrum  coincident.  It  is  obvious  that 
for  this  test  the  surfaces  of  the  ground  glass  and  the 
sensitive  plate  must  fall  into  exactly  the  same  plane, 
and  for  this  reason  it  is  recommended  to  focus  upon 
a  piece  of  ground  glass  placed  in  the  actual  plate- 
holder  which  is  to  be  used.  A  trained  observer  can 
form  a  good  estimate  of  the  chromatic  correction  by 
a  visual  inspection  with  a  telescope  eyepiece,  but 
the  novice  will  do  well  to  trust  nothing  but  a  photo- 
graphic test. 

Curvature  of  Field.  —  Curvature  of  field  is  easily 
discovered  by  examination  of  the  image  of  a  flat  wall 
on  which,  if  possible,  some  printed  pages  should  be 
fixed.  Test  charts  are  not  good  for  this  purpose,  as, 
being  usually  small,  they  require  a  longer  camera 
extension,  and  this  naturally  increases  the  covering 
power  of  the  lens.  Consequently,  a  better  perform- 
ance can  be  obtained  from  a  chart  at  two  feet  than 
from  a  wall  forty  feet  away.  If  a  flat  object  be  not 
available  any  distant  one,  such  as  a  weather-cock  or 
a  chimney,  will  serve.  This  should  first  be  focused 


68  HOW  TO   CHOOSE 

upon  the  centre  of  the  screen,  and  the  camera  rotated 
upon  the  tripod  until  the  image  falls  near  the  margin 
of  the  screen.  The  amount  of  racking-in  which  is 
necessary  is  a  measure  of  the  curvature  of  the  field. 
In  the  case  of  some  anastigmats,  it  is  not  safe  merely 
to  compare  the  image  formed  at  the  centre  of  the 
field  with  that  formed  at  the  margin,  as  the  field  fre- 
quently has  a  dip  extending  in  a  circular  zone,  after 
which  it  recovers  its  original  flatness.  We  have 
used  an  expensive  modern  lens,  which  at  the  same 
aperture  gave  better  definition  at  the  corners  of  a 
6i  X  8|  plate  than  it  did  upon  the  4X5  size.  It  is 
very  necessary  to  see  that  the  front  and  back  of  the 
camera  are  absolutely  parallel,  as  any  swing  or  in- 
clination will  vitiate  this  test. 

Flare  Spot.  —  Flare  spot  occurs  in  two  forms,  one 
being  a  decided  disc  which  appears  at  a  distance  from 
the  centre  of  the  plate  corresponding  to  that  of  any 
bright  object,  such  as  a  gas  flame  or  a  window  which 
may  be  in  the  field,  while  the  other,  which  usually 
occurs  in  single  lenses,  takes  the  form  of  an  ill-defined 
central  patch  of  fog.  The  former  is  easily  discovered 
by  focusing  a  gas  flame  placed  in  front  of  a  dark 
background,  when,  upon  rotating  the  camera,  the 
image  of  the  flame  and  the  disc  will  be  seen  to  ap- 
proach and  recede  from  each  other.  Many  useful 
lenses  suffer  more  or  less  from  this  defect,  which 
appears  only  under  trying  conditions.  The  other 
form  is  more  difficult  to  detect  upon  the  ground 
glass,  but  may  be  found  by  taking  a  bright  sky 
against  a  dark  mass  of  foliage.  If  a  patch  more  or 
less  circular  in  form  is  visible  in  the  centre  of  the 
subject,  it  is  probably  due  to  the  flare. 


AND   USE  A  LENS.  69 

Mechanical  Defects.  —  It  is  now  rare  to  find 
lenses  suffering  from  mechanical  defects,  except  those 
due  to  accident.  If  a  lens  is  badly  centered,  that  is 
to  say,  if  all  the  axes  of  all  its  components  are  not  in 
the  same  line,  the  definition  will  be  affected.  This 
may  be  due  to  careless  workmanship  or  to  the  position 
of  the  lenses  having  been  altered  by  a  fall.  It  may 
be  detected  by  focusing  a  candle  flame  at  a  little 
distance  from  the  centre  of  the  field,  and  then  gently 
rotating  the  lens  in  its  flange.  If  the  lenses  are  per- 
fectly centered,  the  image  will  remain  stationary;  if, 
not,  it  will  move  in  a  circular  direction.  If  the  cells 
are  separately  rotated  in  the  same  way,  it  will  be 
seen  in  which  the  fault  is  present.  Another  test  can 
be  made  without  a  camera  by  holding  the  lens  be- 
tween the  eye  and  a  gas  flame,  so  that  a  number  of 
small  images  of  the  flame  are  seen  inside  the  lens. 
In  a  properly  centered  lens  these  can  be  brought  into 
a  straight  line,  but  if  one  or  more  cannot  be  made  to  do 
so,  it  shows  that  one  at  least  of  the  glasses  is  displaced. 

Defective  glass  sometimes  gives  rise  to  poor  defi- 
nition, and  so  does  too  tight  a  cell.  These  can  be 
detected  only  with  the  aid  of  polarized  light,  which 
shows  any  strain,  as  in  badly  annealed  or  compressed 
glass,  as  dark  patches.  Veins  in  the  glass  may  be 
seen  by  holding  the  lens  up  to  a  small  gas  or  candle 
flame,  so  that  the  surface  is  uniformly  luminous.  The 
streaks  will  then  show  quite  plainly.  Care  must  be 
taken  not  to  confuse  them  with  any  streaks  which 
may  have  their  origin  in  the  eye  of  the  observer.  If 
the  lens  is  turned,  a  genuine  vein  will  turn  with  it, 
but  a  spectral  one  remains  stationary.  Air  bubbles 
of  any  size  which  any  manufacturer  will  pass,  are 


70  HOW  TO  CHOOSE 

of  no  importance,  and  may  be  disregarded.  Being 
magnified  by  the  lens,  they  are  actually  much  smaller 
than  they  appear. 

Choice  of  Focal  Length.  —  Most  cameras  are  fur- 
nished with  a  lens  having  a  focal  length  about  equal 
to  the  diagonal  of  the  plate,  or  a  little  shorter.  This 
is  unquestionably  the  best  all-around  length  for 
general  amateur  photography  outdoors.  There  are, 
however,  a  great  many  subjects  which  are  included 
in  a  much  narrower  angle  of  view  than  such  a  lens 
gives.  For  example,  a  6-inch  anastigmat  on  a  4  X  5 
plate  includes  about  56°  on  the  diagonal  or  about  45° 
on  the  longer  side  or  base.  The  real  picture,  as  seen 
by  the  eye,  seldom  includes  much  more  than  30°,  so 
that  the  print  (or  enlargement)  may  often  include  only 
a  small  portion  of  the  negative.  It  is  far  more  con- 
venient, with  any  camera  except  a  reflex,  to  use  the 
lens  with  45°  angle,  trimming  the  prints,  than  to  use 
a  long-focus  lens  which  includes  30°  on  the  base. 
The  reason  is  that  a  short-focus  lens  has  so  much 
greater  depth  of  field  that  it  can  be  used  successfully 
with  a  focusing  scale,  whereas  the  long-focus  lens 
(9-inch  for  4X5)  would  need  more  careful  focusing 
and  the  use  of  a  smaller  stop  to  obtain  similar  depth. 
We  find  that  a  6-inch  answers  well  for  general  work 
if  great  care  is  taken  to  keep  clear  of  the  temptation 
to  work  for  too  large  an  image.  If  the  image  is  too 
small,  provided  only  it  is  sharply  focused,  it  will 
stand  enlarging  to  at  least  four  diameters  without 
looking  at  all  blurred.  One  can  often  utilize  a  piece 
of  a  4X5  negative,  measuring  from  i  X  i^  to 
2^  X  3  inches,  throwing  it  up  to  10  X  12  or  n  X  14, 
and  still  have  a  picture  sharp  enough  for  framing. 


AND   USE  A  LENS.  71 

Practically,  then,  a  short-focus  lens  has  a  great  ad- 
vantage. The  ideal  way,  naturally,  is  to  have  a 
"  battery  "  of  lenses  of  different  focal  lengths,  so  that 
when  once  the  proper  station-point  for  the  lens  has 
been  found,  a  selection  may  be  made  so  as  exactly 
to  fit  the  picture  to  the  plate. 

Focal  Length  and  Perspective.  —  Strictly  speaking, 
the  perspective  of  the  picture  is  determined  by  the 
point  at  which  the  lens  is  placed.  When  the  de- 
cision is  made  to  photograph  a  certain  scene,  the  eye 
generally  concentrates  on  an  angle  of  about  30°.  If, 
now,  the  picture  is  made  from  the  position  from 
which  the  view  appears  right  to  the  eye,  using  a 
45°  lens,  the  result  will  almost  invariably  appear 
unsatisfactory.  The  principal  object  seems  too  small 
and  too  far  away,  and  there  is  altogether  too  much  of 
the  neighborhood  included  in  the  margins.  As  already 
stated,  the  remedy  is  to  enlarge  the  part  which  con- 
tains what  is  wanted;  but  most  workers  are  inclined 
to  go  closer  the  next  time,  so  as  to  secure  a  larger 
image  for  contact  printing.  The  result  is  that  the 
foreground  objects  appear  too  large  and  distant  ob- 
jects too  small.  If,  on  the  contrary,  one  always  used 
a  9-inch  lens  for  a  4  X  5  negative  there  would  be  a 
constant  incentive  to  move  farther  away  from  the 
subjects,  to  make  sure  of  getting  them  all  in.  To 
some  extent,  this  tendency  would  counterbalance 
the  smaller  "  depth  "  of  the  9-inch  lens;  but  when 
larger  sizes  than  5X7  were  used,  it  would  be  found, 
in  practice,  that  30°  lenses  would  require  considerable 
stopping  down. 

Readers  whose  interests  are  chiefly  along  the  lines  of 
artistic  portraiture  and  landscape  work  will,  however, 


72  HOW  TO  CHOOSE 

find  it  advisable  to  have  at  least  one  long-focus  lens. 
Outside  of  commercial  interiors  and  exteriors  in 
narrow  streets,  it  is  safe  to  lay  down  as  a  principle 
that  the  most  generally  useful  lens  for  a  given  plate 
is  one  of  a  focus  equal  to  the  diagonal;  for  pictorial 
work,  equal  to  the  sum  of  the  two  sides ;  and  for  special 
work  of  a  telephoto  nature,  one  equal  to  twice  the  long 
side,  or  base.  For  4X5,  these  focal  lengths  would  be 
6J,  9,  and  10  inches;  for  5  X  7,  8 J,  12,  and  14  inches. 

Any  one  of  these  three  focal  lengths  will  give  the 
same  perspective  if  all  are  used  from  the  same  station- 
point;  but  the  usable  part  of  the  picture  will  be  con- 
tained in  different  parts  of  the  plate.  We  confess  to 
a  liking  for  using  long-focus  lenses,  composing  the 
picture  on  the  ground  glass  with  the  lens  which  gives 
the  exact  composition  over  the  entire  plate,  and  en- 
larging the  whole  plate;  yet  we  have  had  exactly  as 
good  results  by  enlarging  a  part  made  with  the  45° 
lens.  The  whole  secret  is  knowing  where  to  stop  in 
advancing  towards  the  subject. 

Perspective  in  Portraiture.  —  We  invariably  advise 
that  no  portrait  should  ever  be  taken  with  the  lens 
any  nearer  the  head  of  the  sitter  than  8J  feet.  A 
greater  distance  is  advantageous,  for  if  some  part  of 
the  figure  is  brought  nearer  the  lens  than  the  main 
mass  of  the  body,  it  will  appear  too  large  unless  the 
station-point  of  the  lens  is  far  away.  Amateurs,  how- 
ever, generally  try  to  get  very  close  in  order  to  make 
a  head  of  good  size,  and  their  pictures  lack  likeness 
because  the  features  are  distorted  by  the  too-close 
viewpoint.  If  the  size  of  the  room  will  permit,  the 
half  of  a  symmetrical  lens  (generally  of  the  longer 
focus  recommended  above,  that  is,  twice  that  of  the 


AND   USE  A  LENS.  73 

complete  lens)  will  be  found  best  for  portrait  work,  as 
the  loss  of  speed  is  compensated  for  by  the  better  per- 
spective and  increased  depth  of  sharp  field.  Even  if 
the  longer-focus  lens  is  used  at  the  same  distance,  it  will 
give  the  same  perspective  but  a  larger  head.  When 
it  is  used  from  twice  the  distance  to  get  the  same  size 
head,  its  advantage  is  tremendous,  for  the  perspective 
is  much  more  natural  and  pleasing  and  all  parts  of 
the  figure  can  be  focused  at  once  without  excessive 
use  of  the  vertical  and  horizontal  swings.  Thus  one 
avoids,  for  example,  having  the  nearer  shoulder  too 
large  and  out  of  focus  and  the  farther  shoulder  a 
shapeless  and  detailless  blur. 

Equivalent  Focal  Length.  —  The  terms,  focus,  focal 
length,  and  equivalent  focus,  as  well  as  equivalent 
focal  length,  are  frequently  met  with  in  descriptions 
of  lenses.  The  correct  term  is  equivalent  focal  length, 
although  the  other  three  are  often  used.  A  simple 
lens  of  the  double  convex  type,  with  both  curves 
alike,  has  an  easily  measurable  focal  length,  this  being 
the  distance  from  the  centre  of  the  lens  to  the  image, 
when  focus  has  been  obtained  on  some  object  distant, 
say,  200  times  the  focal  length.  With  other  types  of 
lenses,  the  optical  centre  does  not  coincide  with  the 
physical  centre.  Consequently  since  the  distance 
from  the  centre  of  the  lens  to  the  centre  of  the 
image  cannot  be  accepted  as  the  focal  length  of  the 
lens,  we  must  find  some  easily  measurable  distance 
to  which  the  focal  length  is  equivalent.  It  is  geomet- 
rically true  that  when  two  lenses  are  used  under 
identical  conditions,  if  they  produce  images  of  the 
same  size  their  focal  lengths  are  equivalent.  If,  then, 
a  simple  lens  and  a  complex  lens  yield  images  of 


74  HOW  TO  CHOOSE 

equal  sizes,  when  used  under  identical  conditions, 
their  focal  lengths  must  be  equivalent.  As  the  focal 
length  of  the  single  lens  is  directly  measurable,  the 
equivalent  focal  length  of  the  complex  lens  is  known 
immediately.  Consequently,  if  the  single  lens  has  a 
focal  length  of  6  inches,  the  complex  lens  will  have 
an  equivalent  focal  length  of  6  inches. 

A  simpler  conception  of  this  may  be  had,  if,  instead 
of  a  single  lens,  we  visualize  in  its  place  an  ordi- 
nary pinhole,  whose  "  focal  length  "  may  be  con- 
sidered as  the  distance  from  the  pinhole  to  the  centre 
of  the  image.  The  same  reasoning  which  applied  to 
the  single  lens,  applies  in  the  case  of  the  pinhole,  and 
there  is  no  ambiguity  as  to  the  path  of  the  light  rays, 
to  confuse  the  beginner,  if  the  position  of  the  optical 
centre  of  the  single  lens  is  not  entirely  clear  to  him. 
For  practical  purposes  it  may  be  said  that  the  equi- 
vocal focal  length  of  a  lens  is  the  same  as  that  of  a 
single  lens  or  pinhole  producing  an  image  of  equal 
size  under  identical  conditions. 

Back  Focus.  —  In  purchasing  a  lens  for  a  reflecting 
camera,  it  is  sometimes  important  to  know  the  back 
focus,  or  distance  from  the  rear  lens  cell  to  the  ground 
glass  when  the  lens  is  focused  for  far  objects,  in 
order  that  we  may  be  sure  it  will  clear  the  mirror. 
Some  anastigmats  have  a  back  focus  considerably 
shorter  than  the  equivalent  focal  length  and  some 
kinds  have  one  considerably  longer.  One  6-inch  lens 
may  have  a  back  focus  of  about  4!  inches  and  so 
can  be  fitted  to  a  sJ  X  4!  reflex,  but  scarcely  to  a 
4X5;  while  another  make  may  have  a  back  focus 
of  about  7  inches  for  a  6-inch  lens. 

Circle  of  Illumination.  —  Although  a  lens  is  often 


AND   USE  A  LENS.  75 

chosen  because  it  can  be  used  as  a  wide-angle  ob- 
jective on  a  larger  camera,  it  may  sometimes  fail  to 
give  clear  negatives  when  used  on  the  smaller  camera. 
The  reason  is  that  the  large  circle  of  illumination 
covers  the  walls  of  the  bellows,  from  which  great 
volumes  of  light  are  reflected  to  the  plate,  fogging  it 
all  over.  The  trouble  is  most  pronounced  in  focal- 
plane  folding  cameras  which  are  not  fitted  with  the 
accordion  type  of  bellows,  but  present  flat  surfaces  of 
considerable  area  acting  as  mirrors  to  cast  the  light 
on  the  plate.  For  this  reason,  a  reflecting  camera 
often  works  better  with  a  lens  incapable  of  covering 
sharply  a  plate  more  than  one  size  larger  than  it  is 
rated  for;  and  wiseacres  who  think  every  lens  should 
have  great  reserve  covering  power  are  often  con- 
founded by  the  superior  work  of  what  they  deem  an 
inferior  lens.  Many  complaints  that  air-space  lenses 
give  foggy  images  are  due  to  reflected  light  from  the 
bellows.  The  trouble  can,  of  course,  be  diminished 
by  reblacking  the  camera  interior  with  a  really  dead 
black  paint  and  supplying  a  deep  lens  hood  to  cut 
off  all  scattered  light,  even  if  the  hood  cuts  down  the 
circle  of  illumination.  No  harm  will  be  done  so  long 
as  it  does  not  interfere  with  covering  the  plate  in  use. 
Focusing  with  the  Swingback.  —  Although  depth 
of  field  is  easily  secured  by  stopping  down,  users  of 
plate  cameras  provided  with  vertical  and  horizontal 
swings  have  a  tremendous  advantage  over  the  film- 
camera  devotee.  The  swings  can  be  used  in  two 
ways.  The  first,  and  most  obvious,  is  to  allow  one 
to  keep  the  back  always  vertical,  even  if  the  camera 
is  pointed  up  or  down.  This  movement  is  invaluable 
in  architectural  work,  for  a  vertical  line  can  be  ren- 


76  HOW  TO  CHOOSE 

dered  as  such  only  if  the  surface  receiving  the  image 
is  vertical.  The  second  way,  far  more  useful,  is  to 
allow  the  back  to  be  moved  to  bring  an  image  into 
focus  without  stopping  down.  Assuming  a  6-inch 
lens,  the  image  of  an  object  several  hundred  yards 
away  is  formed  6  inches  from  the  lens,  but  that  of 
an  object  6  feet  away  will  lie  about  6f  inches  away. 
If,  now,  the  camera  is  tilted  down  towards  the  near 
foreground  object  and  the  vertical  swing  is  used  to 
set  the  top  of  the  ground  glass  two-thirds  of  an  inch 
behind  the  point  where  the  distant  object  appears  on 
it,  both  images  can  be  focused  with  a  moderately 
large  stop.  On  the  contrary,  if  the  back  were  kept 
in  its  normal  position,  the  use  of  a  very  small  stop 
would  become  necessary  to  secure  a  sharp  image  of 
both  at  once.  The  horizontal  swing  is  equally  useful 
in  countless  instances,  particularly  when  near  objects 
occur  at  one  side  and  not  at  the  other,  or  to  focus. a 
diagonal  line,  as  of  a  procession,  throughout  its  depth. 
In  fact,  so  useful  are  the  swings,  rising  and  falling 
front,  and  other  adjustments  of  a  good  form  of  plate 
camera,  that  one  can  get  the  utmost  out  of  a  lens,  in 
a  difficult  situation,  only  by  knowing  how  to  take 
full  advantage  of  every  movement.  The  lack  of  ad- 
justments is  the  most  serious  drawback  of  the  reflex 
camera,  as  well  as  of  the  roll-film  camera.  Still,  the 
use  or  non-use  of  these  conveniences  depends  largely 
on  the  class  of  work  one  attempts,  and  the  simplest 
box  camera  will  handle  a  surprisingly  large  variety 
of  work  if  intelligently  used. 

Supplementary  Lenses.  —  The  owner  of  only  one 
camera  often  finds  that  he  would  like  to  do  some 
kind  of  work  for  which  his  lens  is  not  of  quite  the 


AND   USE  A  LENS.  77 

right  focal  length.  For  instance,  to  get  a  large 
head  in  a  portrait,  the  use  of  a  lens  having  a  focal 
length  of  about  3^  feet,  placed  in  front  of  the  rapid 
rectilinear  lens  like  a  lenscap,  will  allow  the  camera  to 
be  brought  to  3!  feet  from  the  head.  This  results 
in  nearly  sharp  focus  and  a  good-sized  head,  as  the 
supplementary  lens,  called  a  "  portrait  attachment," 
brings  the  image  to  a  focus  without  requiring  the 
camera  front  to  be  moved  from  its  loo-foot  position. 
In  the  same  manner,  lenses  of  greater  or  lesser  focus 
can  be  utilized  to  alter  the  focus  of  the  lens  and 
obviate  the  need  of  a  long  bellows.  Supplementary 
lenses  are  often  sold  in  sets,  consisting  of  portrait, 
wide-angle,  copying,  and  telephoto  lenses,  with  a 
"  duplicator  "  and  a  cheap  glass  ray  filter  added.  By 
purchasing  such  a  set,  the  owner  of  a  short-bellows 
camera  can  get  some  of  the  effects  which  otherwise 
would  need  a  long-bellows  camera.  Spectacle  lenses 
can  also  be  utilized.  The  rule  is  to  place  before  the 
lens  a  lens  having  a  focus  equal  to  the  distance  from 
lens  to  subject.  Simple  lenses  introduce  aberrations 
into  an  optical  system,  so  a  medium  or  a  small  stop 
must  often  be  used  to  secure  satisfactory  definition. 

Utilizing  a  Curved  Field.  —  Although  curvature  of 
field  is  a  lens  defect,  it  is  possible  at  times  without 
stopping  down,  to  avoid  the  natural  poor  quality  of 
definition  of  an  uncorrected  lens.  For  instance,  when 
making  a  negative  of  a  group  of  people,  who  can  be 
arbitrarily  arranged  at  different  distances  from  the 
lens,  their  images  can  be  brought  into  equally  sharp 
focus  on  the  plane  of  the  plate,  by  arranging  them  in 
an  arc  of  a  circle,  the  arrangement  being  dictated  by 
what  one  sees  on  the  ground  glass. 


HOW  TO   CHOOSE 


Table  of  Hyper 


F 

It 

*e3  c 

N 

w~g 

A  =  f  value  of  lens 

§  ^ 

II 

w 

1.9 

2.2 

3.5 

4.2 

4.5 

5.4 

5.6 

6.3 

6.8 

7 

7.7 

2 

¥77 

70 

61 

38 

32 

30 

25 

24 

21 

20 

19 

17 

3 

¥77 

158 

136 

86 

71 

67 

56 

54 

48 

44 

43 

39 

3l 

777 

161 

139 

88 

73 

68 

57 

55 

49 

45 

44 

40 

4 

577 

211 

182 

114 

95 

89 

74 

63 

59 

57 

52 

4s 

707 

145 

121 

113 

94 

90 

80 

74 

72 

66 

5 

577 

149 

124 

116 

96 

93 

83 

77 

74 

68 

5^ 

577 

180 

I5O 

140 

117 

IOO 

93 

'  90 

82 

6 

577 

214 

179 

167 

139 

134 

119 

no 

107 

97 

6^ 

577 

252 

210 

196 

163 

157 

140 

129 

126 

114 

7 

577 

292 

243 

227 

189 

182 

162 

150 

146 

133 

8 

577 

305 

254 

237 

198 

190 

169 

156 

152 

139 

9 

577 

386 

32I 

300 

250 

241 

214 

199 

193 

175 

10 

T77 

357 

298 

278 

231 

223 

198 

184 

179 

162 

u 

4j 

288 

240 

224 

187 

1  80 

1  60 

148 

144 

131 

12 

343 

286 

267 

222 

214 

190 

176 

171 

156 

13 

77 

252 

235 

196 

189 

168 

155 

137 

14 

7*7 

292 

272 

227 

219 

194 

1  80 

175 

159 

15 

¥ 

335 

313 

260 

251 

223 

207 

201 

183 

16 

356 

296 

286 

254 

235 

229 

208 

17 

77 

430 

401 

334 

323 

287 

266 

258 

235 

18 

7*7 

482 

450 

375 

362 

321 

298 

289 

263 

19 

71? 

537 

501 

418 

403 

358 

332 

322 

293 

20 

1 

595 

556 

463 

446 

2Q7 

368 

21 

77 

656 

O  0 

613 

T-^O 

510 

492 

oy  / 

437 

o  ^ 

405 

394 

358 

22 

7*7 

720 

672 

560 

540 

480 

444 

432 

393 

2? 

JL 

787 

73  C 

612 

r  2  C 

486 

o 
24 

X 

II 

857 

1  OO 

800 

666 

643 

•J  ^  O 

571 

529 

5H 

467 

This  table  shows  the  nearest  point  in  sharp  focus  when  objects 
infinitely  distant  are    focused  on.      It  is  calculated    from  the 

F2 
formula   hf= .      — ,   where  hf  =  hyperfocal  distance  in 

12  /\  A    /\  U 

feet,  u  =  diameter  in  inches  of  the  circle  of  confusion  or  greatest 
allowable  unsharpness  of  the  image  of  a  point,  F  =  the  equiva- 
lent focal  length  of  the  lens  in  inches,  and  A  =  the  /  number. 
This  table  is  usually  calculated  for  a  single  circle  of  confusion 


AND  USE  A  LENS 


79 


focal  Distances 


A  =f  value  of  lens 


8 

9 

11.3 

12.5 

16 

18 

22.6 

25 

32 

36 

45.2 

50 

64 

17 

15 

12 

II 

8* 

7* 

6 

si 

4 

3i 

3 

2| 

2 

38 

33 

27 

24 

19 

17 

13 

12 

9\ 

8| 

6| 

6 

41 

38 

34 

27 

25 

19 

17 

14 

12 

91 

8£ 

6! 

61 

4^ 

SO 

44 

35 

32 

25 

22 

18 

16 

13 

ii 

9 

8 

6; 

63 

56 

45 

41 

32 

28 

22 

20 

16 

14 

ii 

10 

7} 

65 

58 

46 

42 

33 

29 

23 

21 

1  6 

IS 

12 

10 

8| 

79 

70 

56 

50 

39 

35 

28 

25 

20 

18 

14 

13 

10 

94 

83 

66 

60 

47 

42 

33 

30 

23 

21 

17 

15 

12 

no 

98 

78 

70 

55 

49 

39 

35 

28 

25 

20 

18 

14 

128 

H3 

90 

82 

64 

57 

45 

4i 

32 

28 

23 

20 

16 

133 

119 

94 

85 

67 

59 

47 

43 

33 

30 

24 

21 

17 

169 

150 

119 

1  08 

84 

75 

60 

54 

42 

38 

30 

27 

21 

156 

139 

no 

IOO 

78 

69 

55 

So 

39 

35 

28 

25 

2O 

126 

112 

89 

81 

63 

56 

45 

40 

32 

28 

22 

20 

16 

150 

133 

1  06 

96 

75 

67 

53 

48 

38 

33 

27 

24 

19 

132 

117 

93 

85 

66 

57 

47 

42 

33 

29 

23 

21 

17 

153 

136 

1  08 

98 

77 

68 

54 

49 

38 

34 

27 

25 

19 

176 

156 

124 

H3 

88 

78 

62 

56 

44 

39 

31 

28 

22 

200 

I78 

141 

128 

IOO 

89 

7i 

64 

So 

44 

35 

32 

25 

226 

2OI 

1  60 

144 

H3 

IOO 

80 

72 

56 

So 

40 

36 

28 

253 

225 

179 

162 

127 

H3 

90 

81 

63 

56 

45 

41 

32 

282 

251 

199 

181 

141 

125 

IOO 

90 

7i 

63 

So 

45 

35 

312 

278 

221 

200 

156 

139 

no 

IOO 

78 

69 

55 

50 

39 

344 

306 

244 

220 

172 

153 

122 

no 

86 

77 

61 

55 

43 

378 

336 

267 

242 

189 

168 

J34 

121 

95 

84 

67 

61 

47 

413 

367 

292 

265 

207 

184 

I46 

132 

103 

92 

73 

66 

52 

450 

4OO 

318 

288 

225 

200 

159 

144 

H3 

IOO 

80 

72 

56 

inch)  for  all  focal  lengths,  but  as  the  definition  required 
for  motion  pictures  or  small  negatives  for  enlargement  made 
with  short-focus  lenses  is  much  greater  than  that  needed  for 
portraits,  the  value  of  u  taken  for  each  focal  length  is  given. 
If  the  lens  is  focused  on  objects  at  the  distance  given  in  the  table, 
the  sharpness  will  be  satisfactory  between  half  that  distance  and 
infinity! 


8o 


HOW  TO  CHOOSE 


Reducing  and  Enlarging  Tables 


All  fig- 
ures in 
table]are 
in  ins. 


Reductions 


Samel     *     I    .*     I    .*     I    .4     I    .*     I    .*     I    .*     I    .*     I    ft    I    A    I    £ 
size    |  size  I  size     size  |  size  I  size     size     size  |  size     size     size  I  size 


Focus 
of  lens 
used 


Enlargements 


Same     2         3         4         5   fc 
size   times  times  times  times 


6 

61 

7 

8 

9 
10 
11 
12 


6 

6 
7 
7 
8 
8 
9 
9 

10 
10 
11 
ii 
12 

12 

13 

13 
14 
14 
16 
16 
18 
18 
20 


22 


24 

24 


12 

6 

6i 
15 

7j 

8| 
18 


21 

242 

12 

27 

i3l 
30 
i5 
33 


36 

18 


12 

4 
14 

163 

si 

18 

6 
20 

6| 
22 

7i 

24 

8 

26 

283 

9i 
32 

iof 
36 

12 

40 

i3i 

44 
i4l 
48 
16 


15 
3l 


4f 
20 

5 
22| 

s! 

25 

6| 

61 
30 

7i 


35 


40 


45 


50 


55 


60 

15 


18 

3l 
21 

4i 
24 

4f 
27 

5! 
30 

6 
33 

6f 
36 

7* 
39 

7* 
42 

8f 
48 

9-! 

54 
60* 

12 

66 

13* 
72 


6 


times  times  times  times  times  times  times 


21 


28 

4l 
31* 

si 

35 

5f 
38| 

42 

7 


49 

8^ 
56 

63 
70 
77^ 

84 £ 

14 


24 

3f 
28 

4 
32 

4* 
36 

5* 
40 

5f 

44 

6? 
48 

6f 
52 

7f 
56 


64 


72 


80 


88 


96 


3U 


36 


40 


45 


49^ 


54 


7r5e 
63 
7l 
72 


81 


90 


99 


108 

13* 


9 


30 

3l 
35 

3t 
40 

4t 
45 

5 
50 

5-i 

55 

60 9 

6| 
65 

7i 
70 


80 


90 


100 


110 


120 


10 

ime 

33^ 


44 

4T4o 


55 

si 


66 


71* 

72% 

77 


88 


99 


110 


121 


132 


11 


36 

3T8r 
42 
3i9r 
48 

64 

4H 

60 

5i5T 

66 

6 

72 


12 


39 


451 


52 


65 


5lf 
78 
61 


78 

84 ' 

7T7T 

96 

8T8T 
108 

9T9T 

120 

iof 
132  143 


144 


104 


117 

9l 

130 


156 

13 


Bold  figures  are  distances  of  lens  from  easel  in  enlarging,  or 
from  lens  to  photo  being  reduced  in  copying.  Light  figures  are 
distances  from  lens  to  negative  being  enlarged  or  camera  exten- 
sion in  case  reduced  size  copies  are  being  made.  The  outer  end 
of  lens  (cap  end)  should  face  bromide  paper  in  enlarging  and  in 
reducing  should  face  object  being  copied.  Distances  are  meas- 


AND  USE  A  LENS  81 

ured  from  nodal  points,  not  diaphragm  of  lens,  and  while  measur- 
ing these  distances  from  diaphragm  will  give  satisfactory  results 
in  many  cases  when  enlarging  with  large  apertures  or  at  great 
distances  final  focusing  should  be  done  by  inspection.  Data 
not  given  in  the  table  may  be  calculated  as  follows: 

Conjugate    Foci.  —  Let    u  =  distance   of   object   from   lens, 
v  =  distance  of  image  from  lens,  F  =  focal  length  of  lens. 


or  F  (u  +  v)  =  uv,     e.g.,     3  (12  +  4)  =  12  X  4- 

If  object  is  reduced  n  times  upon  the  focusing  screen,  u  is 
n  -\-  i  times  the  focal  length  of  the  lens,  and  v  is  the  focal  length 

plus  —of  the  focal  length.     Thus  12  in.  photographed  down  to 
n 

i  in.  with  a  6-in.  lens  gives  u  =  13  X  6,  and  v  =  6  +  (^  X  6) 
=  6i 

Rule  for  Copying.  To  find  distance  from  lens  to  original. 
Multiply  focal  length  of  lens  by  the  number  of  times  of  reduc- 
tion, and  add  one  focal  length  thereto.  To  find  camera  ex- 
tension. —  Divide  focal  length  by  number  of  times  of  reduction, 
and  add  one  focal  length  thereto  (see  tables). 

Rule  for  Enlarging.  To  find  distance  from  negative  to  lens, 
divide  focal  length  by  number  of  times  of  enlargement,  and  add 
one  focal  length  thereto. 

To  find  distance  from  lens  to  paper,  multiply  focal  length  by 
number  of  times  of  enlargement,  and  add  one  focal  length  thereto. 

To  Graduate  Focusing  Scale.     Camera  extensions  for  various 

pF 
distances  are  given  by  the  formula      _     ,  where  p  is  focal  length 

of  lens,  and  F  the  distance.  Thus  20  ft.  with  5-in.  lens,  240  X 
5  --  (240  -  5)  =  5-1- 

In  practice  it  is  most  convenient  to  graduate  focusing  scale 
by  trial,  focusing  on  a  test  object  at  known  distances. 

Combining  Lenses.  To  find  the  focal  length  of  two  lenses 
separated  by  a  short  distance,  multiply  the  focal  lengths  together 
and  divide  by  focal  lengths  added  together,  less  the  distance 
between  the  lenses. 


Resultant  focal  length  = 


82 


HOW  TO  CHOOSE 


To  find  the  focal  length  of  supplementary  lens  necessary  to 
reduce  or  increase  the  focal  length  of  a  given  lens,  multiply  the 
focal  length  to  be  altered  by  the  focal  length  desired,  and  divide 
the  product  by  the  original  focal  length  less  the  final  focal  length. 


/i  equalling  the  original  and  F  the  final  focal  length. 

To  reduce  the  focal  length  use  a  positive  lens. 

To  increase  the  focal  length  use  a  negative  lens  and  calculate 
it  as  minus  in  the  formula. 

The  focal  length  of  a  "magnifier"  —  to  bring  near  objects 
into  focus  —  should  be  equal  to  the  distance  of  the  object. 

Diaphragm  Numbers  and  Uniform  System  Numbers 


/values  
U.  S.  nos  

3-i6 

4 

i 

4-5 

5.6 

2 

6-3 

6.8 

7 

7-7 

8 

9 

Relative  exposures  j 

i 

1 
1.6 

r5 

2 

3-i 

V 

2.9 
4-6 

3 
4-9 

3-75 
6 

6.4 

"i 

There  are  today  two  important  scales  of  diaphragm  numbers 
in  use,  but  as  both  are  expressed  in  the  same  notation  on  all 
modern  lenses  and  shutters  except  some  made  in  the  United 
States,  no  confusion  arises  from  this  source  except  that  some 
diaphragm  numbers  are  not  found  in  all  exposure  tables.  On 
account  of  the  latitude  of  exposure  of  all  plates  and  films,  using 
the  exposure  for  the  nearest  number  given  will  rarely  lead  one 
astray.  The  English  system  of  marking  lenses  uses  /:4  as  a 
unit.  It  is  generally  replaced  on  American  cameras  by  the 
Uniform  system,  in  which  the  same  unit  is  numbered  i.  In 
the  Stolze  system,  used  on  Zeiss  and  other  German  lenses,  the 
unit  is/:  Vio  =  /:3.i6.  The  following  table  compares  the  three 
systems  and  gives  the  relative  exposure  times  for  the  different 
stops.  Opposite  the  caption  "/  values  "  are  shown  the/  numbers 
in  both  the  English  and  Stolze  systems;  beneath  are  listed  the 
equivalent  apertures  in  the  U.  S.  system.  The  figures  showing 
the  relative  exposures  are  computed  for  two  systems,  the  unit 
exposures  being  those  for  / 14  and  /:3-i6  respectively.  The  ex- 


AND   USE  A  LENS  83 

posures  which  bear  the  relation  of  2  to  i  to  each  other  are  printed 
in  boldface  type. 

On  old  lenses  various  other  systems  of  markings  may  be  found. 
Zeiss  lenses  were  once  marked  with  numbers  running  in  reverse 
direction  to  that  given  above,  /  :ioo  being  marked  I  and  /:4-5, 
512.  Later /:5O  was  adopted  as  the  unit,  7:4.5  then  being  num- 
bered 128.  Old  Goerz  and  Dallmeyer  lenses  may  occasionally 
be  found  with  numbers  based  on  /:62,  which  is  numbered  384, 
and  the  numbers  run  down  to  3,  equivalent  to  7:5.5.  Still  other 
systems  may  be  found  on  French  lenses,  but  these  are  so  rarely 
seen  outside  of  that  country  that  it  would  be  useless  to  give 
space  to  them  here.  For  full  details  of  the  relation  of  stops  to 
exposure,  the  reader  is  referred  to  No.  I  of  this  series,  The 
Secret  of  Exposure. 


11.3 

12.5 

16 

18 

22.6 

25 

32 

36 

45.2 

50 

64 

7i 

100 

8 

16 

32 

64 

128 

2^6 

8 

13 

10 

16 

16 

26 

20 
32 

32 

51 

39 
64 

64 

IO2 

78 

128 

128 

205 

156 
256 

256 

410 

ffi 

1024 

What  Lens  Shall  I  Buy? 

THIS  PERPLEXING  QUESTION  is  answered  in  detail  by  our 
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ST.   PAUL  STREET,   ROCHESTER,  N.  Y. 
NEW  YORK          WASHING  TON      •     GHICAQO  SAN  FRANCISCO 


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YOUR  CHOICE  IN  THE  LENS  RACE 

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THE  FAMOUS 

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Eastman  Vest  Pocket  Kodak-    ------        3.75 

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6|X8£  Ic  Tessar  F:4.5 65.00 

4X5  Premo  Film  No.  1  R  R  Lens      -    -    -    -        5.75 
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A  New  Visual  Quality  Lens 

SERIES   IV   DOUBLET 

(75  PER  CENT  CORRECTION) 

EXPERT  OPINIONS: 

"Old  boy,  this  is  not  a  lens;  it  is  a  MAGIC  chunk  of  glass." 
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of  a  soft-focus  lens,  so  that  I  can  conscientiously  recommend  it  and 
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Later  Mr.  Anderson  wrote,  "The  more  I  use  it,  the  better  I  like  it." 


THIS  NEW  LENS  is  offered  as  the  best  possible  compro- 
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firm  drawing  of  detail  will  find  in  this  new  series  exactly  what 
they  have  been  looking  for.  It  is  especially  suitable  for  reflect- 
ing cameras  and  home  portraiture,  on  account  of  its  great  speed 
of/:  4.5.  With  its  75  per  cent  correction,  the  full  aperture 
gives  about  the  same  quality  as  /:  6  of  the  older  series,  and 
f\  5.6  will  be  found  equivalent  to/:  8  of  the  same.  Write  for 
Particulars  of 

THE   VISUAL    QUALITY    LENS 


Pinkham  &  Smith  Company 

Two  Stores: 

288-290  BOYLSTON  ST.         13!  BROMFIELD  ST. 
Boston,  Mass. 

Please  mention  Practical  Photography  when  writing  Advertisers 


SOONER  OR  LATER  EVERY  GRAFLEX  OWNER 
WILL  POSSESS  A  STRUSS  PICTORIAL  LENS 


C,  For  scientific  work  use  an  ana- 
stigmat,  but  for  PICTURES  make 
your  outfits  more  complete  by  own- 
ing a  Struss  Pictorial  Lens.  Unsur- 
passed for  Street  Scenes  requiring 
quick  action  and  speed,  Q  Send 
twenty-five  cents  for  a  beautifully 
illustrated,  intensely  interesting  and 
instructive  BOOKLET  FJ.  This 
amount  will  be  refunded  to  each 
lens  purchaser.  C.  Made  in  the  fol- 
lowing focal  lengths,  in  a  highly 
finished  light  aluminum  mount,  Iris 
Diaphragm. 

SINGLE  LENS,  F  ;  5.5 
9*    12*     15*     18*    21"    Other  sizes 
$15    $20   $25    $30    $35       and  speeds 

FURTHER  PARTICULARS   FROM 

KARL   STRUSS, 
5  W.  31st  St.,  New  York 


NEW  ROSS  LENSES 

Cover  Perfectly  with  Brilliant  Definition  and  Are  Extremely  Rapid 
Adaptable  to  All  Modern  Cameras 

ROSS'   New   "XPRES"   Patent 

F/4.5      FULL  APERTURE      F/4-5 

Working  at  full  aperture.  For  Studio  Portraits  and  Groups'1  and  for  all  work  requiring 
extreme  rapidity.  Exceptionally  Brilliant.  Definition  to  margins. 

i2-inch  "XPRES"  THE  LENS  for  Cabinets  and  Whole-plate  Groups.  Admirable  for 
Reflex  and  Hand  Cameras. 

ROSS'  New  "TELECENTRIC"  Patent 

F/5-4    TWO  SERIES    F/6.8 

Large  Image  with  Short  Camera  Extension.  Critical  Definition  at  full  aperture.  Short- 
ened Exposures  Ideal  for  Sporting  Events.  Inexpensive  Studio  Lens.  Covering  to  Margins. 

ROSS'  New  "  COMBINABLES  "   Patent 

F/II      SINGLES   AND  DOUBLETS      F/S-5 

The  Singles  are  the  most  Rapid  Single  Anastigmats,  perfectly  corrected,  giving  Critical 
Definition  to  the  margins  without  distortion  —  for  Landscapes,  Large  Portraits  and  Groups. 
The  Universal  Doublets  are  specially  suitable  for  Portraits  and  Groups  in  the  Studio  and  all 
outdoor  Photography,  Interiors,  Copying,  etc. 

For  Full  Particulars  see  Ross'  New  Price  List,  sent  free  on  application 
American  Agents 

GEORGE  MURPHY,  Inc. 

57  East  Ninth  Street  ....  NEW   YORK 


Please  mention  Practical  Photography  when  writing  Advertisers 


aWOLLENSAKS 


LET  US 
HELP  YOU 


DECIDE  THAT  ALL  IMPORTANT 
QUESTION 


I 


"WHAT  LENS? 


As  manufacturers  of  the  most 
complete  line  with 

B8 

"A  LENS  FOR  EVERY  PURPOSE" 

our  co-operation,  flavored  with  vast  experience,  will  be  very 
helpful  to  you.  And  all  the  assistance  we  can  give  is  yours 
for  the  asking  without  any  sort  of  obligation  on  your  part. 

LENSES 

Just  consider  these  : 

Velostigmat  Series  I  F  :  6.3 
Velostigmat  Series  II  F :  4.5 
Velostigmat  Series  III  F  :  9.5 
Vinco-Anastigmat  F  :  6.3,  F  :  6.8 
Versar  Portrait  and  View  F :  6 


Vitax  Portrait  F: 3.8 
Vesta  Portrait  F  :  5 
Verito  Diffused  Focus  F :  4 
VoltasF:8 
Wide  Angle  F:  16 


RAY  FILTER 

and  Shutters :    Optimo,  Auto,  Regno  Victo,  Ultro,  Studio. 

Now  —  a  postal  request  —  and  our  beautiful  complete  catalogue  and 
any  special  information  desired,  will  reach  you  by  first  return  mail. 
The  time  to  drop  the  postal  is  TODAY. 


WOLLENSAK  OPTICAL  CO. 

ROCHESTER,  N.  Y. 


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CONCERNING 

PRACTICAL  PHOTOGRAPHY 

FROM  THE  PUBLISHER  TO  THE  READER 


A  study  of  this  number  of  Practical  Photography  will  show  you  the  aim  of  the  series,  which 
is  to  give  in  each  number  all  of  the  necessary  information  for  a  mastery  of  some  small  part 
of  the  domain  of  photography.  It  is  not  the  intention  of  the  Editors  to  include  a  lot  of  use- 
less theories,  but  each  number  will  reflect  the  present  state  of  our  knowledge  of  some  branch 
of  photography,  plainly  told  in  such  a  way  that  the  knowledge  can  be  immediately  put  into 
practice  in  the  workroom  or  the  field.  Each  book  will  be  illustrated  as  liberally  as  a  com- 
plete understanding  of  the  subject  demands,  but  no  pictures  will  be  introduced  merely  be- 
cause of  their  beauty,  or  because  of  their  artistic  value.  The  books  will  be  well  made  from 
the  standpoint  of  typography,  paper,  and  illustrations,  will  be  sewed  to  open  flat,  and  will 
handily  fit  the  pocket.  They  will  be  issued  in  two  bindings,  paper  and  cloth.  The  paper 
edition  will  cost  twenty-five  cents  per  issue,  or  $2.50  by  subscription  for  twelve  numbers. 
The  price  of  the  cloth  edition  will  be  fifty  cents  a  copy,  or  $5.00  for  twelve  numbers.  Some 
description  of  the  first  three  numbers  follows. 

NO.   i.    "THE  SECRET  OF  EXPOSURE."^ 

If  exposure  is  correct,  all  other  steps  in  the  making  of  a  good  negative  are  merely  mechan- 
ical. This  book  contains  a  complete  system  for  obtaining  correct  exposure,  on  all  occasions. 
It  discusses  all  the  factors  governing  exposure,  all  the  conditions  under  which  it  should  be 
made,  and  gives  full  and  explicit  directions  for  obtaining  correct  exposure  under  all  circum- 
stances. Complete  exposure  tables  and  speed  lists  of  all  plates  on  the  American  market 
are  given,  and  the  book  concludes  with  a  clear  discussion  of  the  use  of  exposure  meters.  It 
is  the  most  complete  and  practical  treatise  on  exposure  ever  written. 

NO.  2.    "BEGINNERS'  TROUBLES." 

Though  this  book  would  seem  to  be  intended  only  for  the  novice  in  photography,  it  con- 
tains an  enormous  number  of  practical  hints  of  value  to  every  practitioner.  In  a  way  it  is 
a  manual  of  photographic  work,  for  it  gives  useful  pointers  for  success  in  all  the  departments 
of  photography  for  making  a  perfect  negative  and  a  satisfactory  print  from  it.  Its  primary 
purpose  is  to  show  what  may  be  the  causes  of  failure,  and  to  help  the  reader  to  avoid  these 
causes. 

NO.  3.    "HOW  TO  CHOOSE  AND  USE  A  LENS." 

Without  being  too  theoretical,  this  book  explains  the  reason  for  existence  and  the  particu- 
lar qualities  of  each  type  of  lens  now  in  use  in  photography.  It  tells  exactly  what  each  will 
do,  and  how  it  may  be  used  to  the  best  advantage.  It  particularly  describes  the  more  mod- 
ern varieties  of  lenses,  and  gives  full  information  as  to  the  differences  between  them.  It  tells 
how  to  test  a  lens,  and  answers  many  questions  which  every  user  of  a  lens  is  sure  to  ask. 

NO.  4.    "HOW  TO  MAKE  PRINTS  IN  COLOR." 


These  books  are  for  sale  and  subscriptions  will  be  received  by  dealers  in  photographic 
supplies,  newsstands,  booksellers,  or  the  publishers. 


AMERICAN   PHOTOGRAPHIC   PUBLISHING  CO. 
221   COLUMBUS   AVENUE,   BOSTON,   MASS. 

Please  mention  Practical  Photography  when  writing  Advertisers 


THE  SELECTION  OF  A  LENS    after  you  have  con- 
sidered the  matter  fully  with  due  regard  to  the  question 
of  Convertible  Lenses  versus  all  others,  freedom  from 
air  spaces,  well  balanced  focal  lengths,  maximum  working  aper- 
ture consistent  with  good  depth  of  focus  and  moderate  price 
should  lead  you  gently  and  firmly  to  the  conviction  that 

3iirner~5ieieh 

CONVERTIBLE    ANASTIGMATS    F:6.8 

meet  absolutely  every  requirement  by  coming  nearer  to  the  opti- 
cian's idea  of  a  perfect  lens  and  the  photographer's  wish  for  a 
universal  lens.  Find  a  lens  if  you  can  that  will  equal  the 

TURNER-REICH    ANASTIGMAT 

at  every  point.  Don't  hope  to  buy  one  second  hand  as  they  are 
rarely  sold  by  those  who  own  them. 


Send  for  complete  catalogue  of  Cameras  and  Lenses. 


Gundlach-Manhattan   Optical    Co. 

783A  CLINTON  AVE.,  SO.,   ROCHESTER,  N.  Y. 
Please  mention  Practical  Photography  when  writing  Advertisers 


EVERY  reader  of  this  book  is  sure  to  be  seriously  interested  in  photography.  This 
being  the  case,  he  should  follow  the  progress  of  the  art,  and  keep  continually  in  touch 
with  its  latest  developments.  The  surest  way  for  him  to  do  this  is  to  subscribe  to  one 
or  more  of  the  photographic  magazines,  and  we  should  be  very  happy,  in  case  he  is  not  ac- 
quainted with  them,  to  send  him  sample  copies  of  our  two  monthlies,  AMERICAN  PHO- 
TOGRAPHY and  POPULAR  PHOTOGRAPHY.  Each  of  these  magazines  is  edited 
with  the  idea  of  giving  each  reader  as  much  help  and  service  as  the  Editors  can  possibly 
furnish.  The  answering  of  perplexing  questions,  the  criticism  of  readers'  pictures,  with  an 
idea  of  showing  how  they  may  be  improved,  the  stimulation  of  effort  by  the  awarding  of 
prizes  in  monthly  contests,  the  furnishing  of  information  as  to  how  the  camera  may  be  made 
a  means  of  pecuniary  help,  are  but  a  few  of  the  manifold  ways  in  which  our  Editors  give 
the  readers  service.  A  more  particular  description  of  the  characteristics  of  the  two  maga- 
zines follows,  but  the  best  way  to  learn  about  them  is  to  send  for  a  sample  copy,  which  will 
cost  you  nothing  and  subject  you  to  no  obligation. 

AMERICAN  PHOTOGRAPHY  is  today  the  standard  American  photographic  magazine- 
and  has  for  years  endeavored  to  follow  its  motto  by  "  representing  all  that  its  name  implies." 
It  covers  the  whole  field  of  photography,  for  amateur,  professional  and  commercial  photog- 
rapher, publishing  each  month  numerous  strong  articles  on  the  best  current  practice  and  the 
latest  advances  in  each  of  these  departments.  A  strong  motion  picture  department  is  one 
of  its  specialties;  and  it  continually  endeavors  to  interest  its  readers  in  other  departments 
of  photography  which  will  broaden  their  activities  and  give  them  some  interests  beyond 
the  making  of  ordinary  pictures,  which  may  lose  interest  after  a  while.  Its  departments 
include  "Questions  and  Answers,"  "Practical  Hints,"  "Our  Portfolio"  for  picture  criticism, 
"The  Round  World  Exchange  Club"  for  the  exchange  of  prints,  "The  Market  Place,"  with 
information  as  to  ways  of  selling  pictures,  "Competitions,"  literary  and  pictorial,  "Notes 
and  News"  of  clubs  and  manufacturing  activities.  The  subscription  price  is  $1.50  a  year 
in  the  United  States. 

POPULAR  PHOTOGRAPHY  is  a  magazine  intended  primarily  for  the  less  advanced 
amateur.  While  the  illustrations  of  AMERICAN  PHOTOGRAPHY  come  largely  from 
exhibitions  and  show  the  achievements  of  the  more  distinguished  photographers  of  the  day, 
POPULAR  PHOTOGRAPHY  is  illustrated  solely  from  the  work  of  readers,  of  the  kind 
which  every  amateur  wants  to  take,  and  with  each  print  is  given  a  complete  description  of 
the  exact  method  by  which  it  was  made  and  a  criticism  telling  how  it  might  have  been  im- 
proved; thus  forming  a  guide  whereby  every  reader  can  make  prints  similar  to  those  which 
he  admires  in  the  pages  of  the  magazine.  In  addition  to  these  picture  criticisms,  the  maga- 
zine contains  numerous  short  and  pointed  articles,  most  of  which  are  written  in  response  to 
definite  queries  from  readers  for  information  on  certain  points.  They  thus  reflect  the  needs 
of  the  day,  and  have  proved  extremely  popular.  The  whole  keynote  of  the  magazine  is  to 
be  brief  and  practical,  and  no  long  theoretical  articles  are  published.  It  has  consequently 
proved  very  attractive  to  professional  photographers  as  well  as  the  amateurs  for  whom  it  is 
primarily  designed.  A  monthly  competition  and  some  other  departments  are  run  by  this 
magazine  as  well  as  by  AMERICAN  PHOTOGRAPHY.  The  subscription  price  is  $1.00 
a  year  in  the  United  States. 


AMERICAN  PHOTOGRAPHIC   PUBLISHING  CO. 
440   POPE   BUILDING,   BOSTON,   MASS. 

Please  mention  Practical  Photography  when  writing  Advertisers 


As  right  as 
a  full 
jeweled 
watch. 


The  Vest  Pocket 
Autographic  Kodak 

with  Kodak  Anastigmat  lens,  f.  7.7. 

A  vest  pocket  camera  that  will  really  go  in  the  vest  pocket — com- 
fortably. 

A  lens  that  gives  microscopic  definition  and  has  speed  to  spare,  a 
ball  bearing  shutter  that  works  silently,  accurately,  without  jerk  or  jar. 

"  Autographic  "  of  course.     All  the  folding  Kodaks  now  are. 

In  this  camera  Kodak  simplicity  and  the  utmost  convenience  are  combined 
with  an  optical  quality  that  perfectly  meets  the  requirements  of  those  who 
demand  an  instrument  of  the  highest  type. 

Pictures,  1#  x  2>£  inches. 

Price,  $10.1° 

If  it  isn't  an  Eastman,  it  isn't  a  Kodak. 

Catalogue  free  at  your  dealer's,  or  by  mail. 

EASTMAN  KODAK  CO.,  ROCHESTER,  N.  Y.,  Th.  Kodak  aty. 


Please  mention  Practical  Photography  when  writing  Advertisers 


The  construction 
of  the 

Brownie 

Enlarging 

Camera 


is  so  simple  that  its  equally 
simple  operation  becomes 
a  self-evident  fact. 
Insert  your  negative  at  the  small  end  of  the  camera, 
your  Velox  paper  at  the  other,  expose  to  daylight, 
and  develop  and  fix  in  the  regular  way. 

No  focusing — No  dark-room. 

Signalize  some  of  your  vacation  masterpieces  in  large  prints. 
THE  PRICE. 

V.  P.  Kodak  Enlarging-  Camera,  for  3^  x  5M  enlargements  from  1&  x  2}£  negatives,  $1.75 
No.  2  Brownie  Post  Card  Enlarging  Camera,  for  3%  x  6M  enlargements  from  2%  x 

3}£  negatives,  ---- 1.75 

No.  2  Brownie  Enlarging  Camera,  for  5x7  enlargements  from  2^  x  3X  negatives,  -  2.00 

No.  3  Ditto,  for  6M  x  8K  enlargements,  from  3%  x  4K  negatives, 3.00 

No.  4  Ditto,  for  8  x  10  enlargements,  from  4x6  negatives  (will  also  take  3^  x  5^ 

negatives),       ......------              ...  4.00 

EASTMAN  KODAK  COMPANY, 
ROCHESTER,  N.  Y. 

At  your  dealer's. 

Please  mention  Practical  Photography  when  writing  Advertisers 


The  Kodak  Film  Tank 

has  made  the  dark  room  a  relic  and 
better  negatives  the  rule. 

The  big  link  in  the  Kodak  chain 
of  daylight  all  the  way. 


THE  PRICE 

Brownie  Kodak  Film  Tank,  for  use  with  No.  1,  No.  2  and  No.  2  Folding: 

Pocket  Brownie  cartridges,  complete, $2.50 

Vest  Pocket  Kodak  Film  Tank,  for  Vest  Pocket  cartridges,  complete,     2.50 

2M-inch  Kodak  Film  Tank,  for  use  with  all  Kodak  or  Brownie  cart- 
ridges having  a  film  width  of  2>£  Inches  or  less,  complete,  -  -  3.60 

3K-inch  Kodak  Film  Tank,  for  use  with  all  Kodak  and  Brownie  cart- 
ridges having  a  film  width  of  3>£  inches  or  less,  complete,  -  -  5.00 

5-inch  Kodak  Film  Tank,  for  use  with  all  Kodak  and  Brownie  cart- 
ridges having  a  film  width  of  6  inches  or  less,  complete,  •  6.00 

7-inch  Kodak  Film  Tank,  for  use  with  No.  5  Cartridge  Kodak  or  shorter 

film  cartridges,  complete, -  7.60 


EASTMAN  KODAK  COMPANY, 
ROCHESTER,  N.  Y. 

At  your  dealer's. 

Please  mention  Practical  Photography  when  writing  Advertisers 


You  want  to  keep  your  negatives,  of  course, 
and  you  want  to  keep  them  where  you  can  lay 
your  hands  on  them  without  delay — particularly 
those  containing  autographic  records. 


The  Eastman 
Film  Negative  Album 

will  preserve  your  negatives  against  injury  or  loss 
and  will  provide  the  handiest  kind  of  a  reference 
book  wherein  the  answer  to  such  questions  as 
"When  did  I  take  this?"  "Where  was  this 
taken  ?"  may  be  found  on  the  instant. 

THE  PRICE 

For  100  negatives,  IH*2%, 10.75 

For  100  negatives,  2%  x4%,  or  smaller,  .75 

For  100  negatives,  3K  x  4#.  or  4x5,       ....  1.00 

For  100  negatives,  3%  x  5^,  or  smaller,   -  1.00 

For  100  negatives,  5  x  7,  or  smaller,          ....  1.50 

EASTMAN  KODAK  COMPANY, 

ROCHESTER,  N.  Y. 
At  your  dealer1 1. 

Please  mention  Practical  Photography  when  writing  Advertisers 


Flowers  are  only  one 
of  a  number  of  subjects 
that  lend  themselves 
particularly  to  the 
Kodak  and  the 


Kodak 

Portrait 

Attachment 


Made  with  Kodak  and  Kodak 
Portrait  Attachment. 


If  a  vase  of  flowers  struck. your  fancy,  you  would  not  take 
up  a  position  ten  fett  away  in  order  to  admire  it. 

It's  that  way  with  the  Kodak— the  Kodak  can't  see  all  the 
beauty  until  it  comes  within  close  range. 

The  Kodak  Portrait  Attachment  enables  you  to  work  as  close 
to  your  subject  as  two  feet,  eight  inches  with  the  Folding  Pocket 
Kodaks — near  enough  so  that  little  of  beauty  or  interest  may 
escape  it  even  though  the  subject  be  small  in  size. 

It's  just  an  extra  lens  which,  when  slipped  on  over  the  regular 
lens  equipment,  brings  the  Kodak  in  focus  at  short  range. 

And  it  costs  butffty  cents. 
EASTMAN  KODAK  COMPANY, 

ROCHESTER,  N.  Y. 

At  your  dealer*  i. 


Please  mention  Practical  Photography  when  writing  Advertisers 


We  are  as  eager  to  have  you 
make  good  prints  as  you  are — 
that's  the  reason  for 

VELOX 

a   photographic   paper   that  Jits. 

Use^the  new  Contrast  Vtlox  with  flat  negatives. 

At  your  dealer'*. 

NEPERA   DIVISION, 

EASTMAN  KODAK  CO.,  ROCHESTER,  N.  Y. 


Please  mention  Practical  Photography  when  writing  Advertisers 


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6-month  loans  may  be  recharged  by  bringing  books 

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